Nearly every family of a decent size has at least one relative who's a little... strange. Maybe it's Great-Aunt Enid and her collection of carefully mounted cat skeletons (no one knows where she gets them — they just appear), or second cousin Dolf's extensive research library on famous serial killers, complete with memorabilia he buys off of eBay at outrageous prices. (Those clown paintings he adores are particularly creepy.)
Not much of a problem, usually, as long as one is careful not to get cornered by them at family reunions - but what happens when your family are hereditary rulers of some kind? Kings, Emperors, High Priests, whatever you want to call it, the point is you have power. Power that belongs to your family, and your family only. Sometimes, due to random chance - or not-even-remotely random deliberate action - Great-Aunt Enid or second cousin Dolf ends up with the royal prerogatives.
What follows is a reign of grotesque excess, blood, and terror. Eventually, however, King Dolf or Queen Enid will leave the throne, unless they've done something extraordinary... That should be the end of the problem, right? Not necessarily. In fact, not even probably. Dangerous insanity in the ruling line rarely appears in a single isolated case. Nope. Chances are the whole family line is just as tainted somehow, which means that sooner or later - probably sooner - along will come Queen Enid II and King Dolf III, and the whole mess will start up all over again.
This may continue for a good many decades or even centuries, with each new generation crossing its fingers that they get one of the "good" rulers from the line and not one of the "iffy" bunch. If you are unfortunate enough to get stuck with one of the blood-drenched loonies, one common solution is to go find someone else from the same family who didn't get hit with the crazy stick, or at least not whacked quite so hard, and put them on the throne instead of Enid or Dolf. This is where disgraced half-brothers and exiled princes/princesses come into play. Unless something permanent is done about the family problem, however, this is most likely just a temporary solution. Give it a generation or three and it's back to Queen Enid IV and King Dolf VI.
One not-at-all common solution is to just get rid of the "absolute hereditary power in the hands of a single individual" government setup, but that's easier said than done; switching from monarchy to republic (be it aristocratic or democratic) on the fly is a tricky job best not undertaken by amateurs and perhaps not possible at all depending on your country's power structure, economic setup, or general level of civilization. (A more feasible solution would be the replacement of absolute monarchy with constitutional monarchy, or at least elective monarchy- but somehow nobody seems to think of that outside of real life. Of course, even in the real world it took millennia, plus the right set of legal, cultural, and social conditions, for these ideas to take hold.) Sometimes the problem can be dealt with by swapping out the old family and swapping in some new group, but check the new line carefully for nasty skeletons in the closet before you give them the keys to the kingdom. Generally, though, if you're serious about fixing the "recurring insane monarch" issue, you'll have to figure out what's at the root of the problem and deal with it.
In fiction there are several common reasons why a royal family might be prone to madness.
Genetics: It's In the Blood in the completely literal scientific sense. The issue is strictly genetic. Usually, that means excessive inbreeding, sometimes very excessive. Sometimes, the initial problem wasn't inbreeding but genetic damage by an outside source that was intensified and cemented into the royal line through inbreeding after the fact. In any case, the family just has a crazy streak built in, and you're not getting rid of it unless your society is advanced enough to have genetic engineering (or a magical equivalent thereof) to deal with the problem or egalitarian enough for the high nobility to stop marrying each other. That last bit ought to help, eventually.
Moral Lamarckism is the classic magical version. The moral failings of your forebears express themselves in a taint on your own soul, like a kind of spiritual gene. Functionally, there isn't much difference.
Family Curse: Worse than crappy genes. Someone has cursed the royal line somehow. This can easily be a lot nastier to deal with than a simple problem of bad genes, because even if you're careful about the inbreeding, the curse doesn't care. It may even spread out to people who marry into the royal line and cause them to go mad even though they're only family by marriage, not by blood. It also means that you might not solve the problem by just picking a new family to rule over you - they're likely to get swatted by the curse just as soon as they take power. Obviously, to fix this you need to figure out who or what cursed the royal family and why, and deal with it by whatever means necessary. You could try jumping straight to a parliamentary system and see how the curse deals with having hundreds of "rulers" - but you'd better hope it just doesn't spread out to cover them all or it will make your old problem seem laughably trivial by comparison.
One variant of this is a spiritual imbalance brought on by upsetting the planetary equilibrium. This works just like a curse, but is the result of natural processes rather than deliberate magic.
Cultural: The madness is the product of nurture, not nature, which means exiled princes will be fine, at least for the first generation. If they don't change the culture that produced the madness, it will return. Possible reasons include:
The family has just gotten too used to being pampered and in power, and each generation has gotten a little more corrupt and decadent until finally people started to notice.
The culture expects its rulers to be "divinely touched" and requires the king to be at least a little crazy.
The culture itself is so hard on its rulers that not being paranoid and vicious means your reign will be measured in months, if you're lucky. In this case, you only look insane to cultures outside your own; within your own realm, madness is just a survival strategy.
The very way the royal kids are raised is detrimental to their sanity.
Environmental - Some X-factor specific to the royal family's home location, diet, or environment.
Heavy metal poisoning, especially lead. Seriously; it's a fashion at the moment for forensic archaeologists to imply this as the cause of most of the real world cases of mad monarchs 1500-1815.
Disease. Specifically, syphilis; it's an STD, so it would get passed around the court, it causes madness if untreated, and the treatment wasn't discovered until the 20th century: Salvarsan.
A mysterious food, drink or drug reserved for royal use, with side effects.
They're Just Nuts: anything not covered by the above.
Whatever the reason, your rulers are bonkers, at least as far as objective outside observers are concerned.
Note that royal/imperial insanity is Truth in Television often enough that it can be a bit frightening.
The Caligula is a singular example of this trope, leaving out the familial tendencies, although they arguably applied to him too. In the Blood doesn't apply only to royals, but is one of the many reasons why a royal family can have recurring madness problems.
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A borderline case in Dragon Ball Z: although Frieza is the most well-known member of his family, others are shown, and for the most part, they're just as crazy and evil as he is. Fan theories tend to attribute this trope to why they're essentially Lawful Evil.
One Piece has the World Nobles, otherwise known as the Tenryuubito (or "Celestial Dragons"). In particular, we meet a family of three World Nobles, St. Rosward and his children, St. Charlos and St. Shalulia. Their only claim to influence is being the direct descendants of kings who founded the One Piece-verse's World Government. However, that claim allows them to perform atrocities ranging from shooting people for having the gall to speak to them directly to randomly naming women on the street as the newest additions to their long lines of wives (and sending the ones they're tired of back to the common folk) with absolutely no fear of retribution as the government will sic a Marine Admiral, complete with fleet, on anybody who dares defy them. Not that Luffy cared.
Hell, they're even above monarchs of the countries that encompass the World Government, yet they do no actual governing at all. Lesser nobles of such countries with monarchs of a similar mindset are willing to do anything to impress them in hopes that they will use their authority to up their status as nobles. One notable instance of this was Goa Kingdom, the place where Luffy and Sabo were born, and the place where they and Ace were raised.
Tower of God: Hendo Lok Bloodmadder, head of the noble Hendo family, sacrifices each of his children at the young age of 100 (relatively young for To G standards) to keep himself immortal. The kids are completely fanatic about daddy.
Whereas the family of King Zahard seems to be a bunch of quirky young girls with limited amounts of sanity and common sense.
And, in the Blue Corner, we have the Koon Family. The Head, Koon Eduan, has many wives and many, many resulting children and grandchildren (and, seeing as this is the Tower) great-to-the-power-of-who-knows-how-many grandchildren... The Family Tree must resemble an overgrown mangrove swamp by now. And, they believe in regular attempts at pruning: politics, betrayals, backstabbings and a coming-of-age tradition that's murder on the kids are all parts of their game. This tends to produce schemers and those with a certain yen towards paranoia that others in the Tower are wary of.
An interesting variant: the heir presumptive of the Holy Kingdom of Saillune in Slayers, Prince Phillionel, while somewhat of a Leeroy Jenkins-like lunatic with a passion for physical fights, is probably the sanest and respectable member of the family and a stellar runner of the country; his younger daughter Amelia has most of his traits. The rest of the family is filled with Dirty Cowards who will do anything to move up a spot for the throne, with no concern for others - both of Phil's younger brothers, Christopher and Randionel, and his nephew, Alfred, have attempted to murder him. Alfred in particular contracts one (two in the anime) Mazoku to both take out Phil and take Saillune for himself while letting Christopher (his father) take the blame. In the novels, Christopher himself kills Alfred to prevent any more damage, and nearly commits suicide himself.
Among the craziest is Amelia's older sister, Gracia, otherwise known as Naga the Serpent. She runs off after Phil's wife is murdered, which is NOT helped by how she found her mom dead and bloodily killed the assassin. She then spent years traveling alone (and with Lina) in an attempt to obtain treasure, learn about the world (sort of), drink, and gain fame without much effort (which definitely doesn't work in her favor, especially when she's traveling with Lina). While she is willing to help others, she is like Lina insofar as she does it for her own gain. In the novels, she keeps in contact with Phil, but in other media it seems that she hasn't made contact in years, which disconcerts Amelia.
Saiunkoku Monogatari. Where to even begin? The previous emperor of Saimono deliberately pitted his sons against each other, and then exiled his former favorite, the most competent of the lot. Of the remaining five princes, all but the youngest were killed fighting each other in a Succession Crisis that nearly destroyed the country. The Un Favoriteyoungest son Ryuuki became the heir. Ryuuki was none too keen on this and feigned incompetence and spread rumors about his sexuality to make himself unpopular and get out of producing an heir. Ironically, avoiding the temptations of power and recognizing his own insufficient preparation for the throne made him the Closest Thing We Got to a stable, responsible Emperor. Ryuuki starts to act like a real ruler after his advisors find him a consort capable of acting as a privy councillor. Oh, and that exiled prince? Turns out he's not so exiled after all...
And that's not even getting into the seven other families that make up the ruling class of Saimono. To give a sense of how screwed up they are, the Kou family is currently being lead by Kurou, the youngest of three living brothers, because Shouka and Reishin each tossed the clan leadership like a hot potato as soon as it fell to them. Kurou just hasn't got anyone else to palm it off on, so he gets stuck herding crazy assassination-happy cats.
Vampire Game is all about one princess's dealings with her own extended Royally Screwed-up family, wherein Incest Is Relative is the least screwed-up thing one can encounter, not to mention the Chimeras and of course Royal espionage. Oh, and there's a vampire who wants to kill her, too. This is mostly a comedy.
In Samurai Pizza Cats, Princess Vi is a selfish, spoiled brat who exiles people to Prisoner Island at the drop of a hat, her mother considers firing a rocket launcher at her daughter as an appropriate family greeting, and Emperor Fred... to say he's got a few screws loose is to imply he's got any screws left.
The three Vance sisters from Queen's Blade are so messed up that, if it weren't for the fact that their father seems to be a fairly sane, stable sort, one would think the gods were justified in dethroning their family's ancestors as the former rulers of the known world and replacing the hereditary nobility with the titular tourney, which seems tailor-made to avoid the negative effects that this can have on the political arena.
Firstly, the sisters as a whole are part of an incestuous lesbian Love Triangle, where eldest daughter Claudette has a crush on youngest daughter Elina that goes unrequited (except in the Hide & Seek continuity) because Elina is instead obsessively in love with the middle daughter, Leina. Leina herself is only Ambiguously Gay, but is stable enough that if she is a lesbian, her attraction is to a woman who is not one of her sisters. Also, all three of them have serious mommy issues due to their Missing Mother — Claudette actually gets a double whammy of this, as her mother died before her father married the woman who gave birth to Leina and Elina, and then she died as well.
Claudette has serious issues with her family due to the fact that she's an illegitimate daughter and so not allowed to officially inherit the family estates, despite being the most formidable fighter. She does love her sisters, but at the same time she hates them, and she also feels torn between love & loyalty and hate for her father. In Rebellion, with a little magical coaxing from the Swamp Witch, her negative feelings turn her into a full-blown Evil Overlord, thanks in part to Leina giving her the throne of Queen.
Leina is technically the most stable of the sisters, but still suffers from severe inferiority issues that make her feel unfit to be the heir to the Vance family name and induce her to try and literally run away from her responsibilities.
Elina, finally, is the most unquestionably screwed up of the sisters. A Spoiled Brat who regards all others as beneath her notice and tortured one of her servants to the point she becomes one of the series' Big Bads just for the hell of it, which she did as a child, she has a complete Lack of Empathy and is also an unabashed incestuous Psycho Lesbian, who in the first anime episode is seen vindictively talking about how she'll make Leina's future husband suffer for having the audacity to marry her beloved sister.
Ironically, this pops up in Legend of the Five Rings. Odd for two reasons: One, the new family line had two generations before being wiped out in their entirety. And two, none of them seemed to be genetically crazy, the first emperor went nuts after being kidnapped and tainted, the next because he had way too much magical power, and the third because his sister died, and the evil of the world showed up and asked if he could join the royal court.
Marvel Universe character of antiquity Namor the Sub-Mariner is both an example and a subversion. By all accounts, he rules the kingdom of Atlantis relatively well. However, he is also provably crazy: his unique Atlantean/Human physiology means that he requires both air and water to function properly, and if he goes too long without one or the other, his body chemistry drives him towards excessive rage and dangerous short-sightedness; a very dangerous thing indeed in someone strong enough to fight the Hulk to a standstill.
Also of Marvel, Magneto and his offspring formed a House of M in the miniseries of the same name, but in a subversion (aversion?), the royal family seemed pretty well-balanced. In the Ultimate line, however, the same 'royal family' is... Well, let's just say they've got problems. General explanation? Big Daddy M's crazy-genes, plus power-induced madness.
In the X-Wing Series, royalty of the planet Eiattu interbreed and use technology to keep the line "pure" of the ills afflicting the common folk. But nature abhors a vacuum. Plourr Illo, revealed as the last confirmed survivor of the main royal family after the other nobles had a bloody revolution (her story was loosely based off of the legend of Anastasia), tells the other characters why the man rumored to be her brother (a new kind of revolutionary, this time of the common people) can't be him.
"All those years of dipping from the same genetic pool caused a wrinkle, a flaw in an otherwise normal family line. We set out to keep ourselves above the common man and found ourselves with a thing from the deepest pit of the Sith."
Well, she also knows it's not him because on the night her family was killed, her father managed to get the two of them out and her brother started screaming for the revolutionaries to come and find her and slit her throat so he could be Emperor. So she killed him.
The My Little Pony: Friendship Is Magic fanfiction, Maternal Instinct has the royal family of the Changeling Kingdom, the House of Roachanov. Although Changeling culture is primarily based off Imperial Japan, the royal family has a reputation of intermarriage amongst its members that, throughout generations has led to many physical and mental disabilites and illnesses much more similar to those of old European royalty. Queen Chrysalis appears to have dodged most of these ailments, but her daughter and heir, Crown Princess Pupa is both heavily mentally and physically disabled. Pupa, at the time of the fanfiction, is approximately the same age as the Cutie Mark Crusaders, yet she can neither walk or talk, and is treated as virtually an infant and carried around as one by her carers. She is comparable to King Charles II of Spain and Feodor I of Russia. As one commentator remarked, "the Changeling royalty is essentially a tour of all the screwed up monarchies of Europe.", again ironic as the culture is primarily Japanese.
Let Them Fade is a terrific Harry Potter fic exploring, in the form of a conversation between Snape and an adult Hermione, the results of long-term inbreeding among Purebloods, the wizarding world's analogue to royal inbreeding: "For every Pureblood child in my generation, I have calculated or deduced the existence of five stillbirths or miscarriages." She also points out an increasing number of Squibs and prevalence of learning disorders among the surviving Pureblood children, and calculates that the Death Eater war hastened the fatal genetic bottleneck by 200 years, because it killed off a substantial chunk of the remaining gene pool. She covers these findings up in her official Ministry report but tells Snape in private. That way he can discreetly spread the word to affected families but there won't be any coercive breeding laws based on her discovery.
The Prentiss family in The Manchurian Candidate (known in the novel as Iselin). The novel alludes pretty frankly to incest between Eleanor and her father Tyler, and relates with equal candor at least one instance of same between Eleanor and her son Raymond. While he's under mind control, no less. All three are driven, passionate patriots working at high levels of office — Tyler was a diplomat, Eleanor is a Senator and Raymond is a Representative running for Vice President. Over the course of his campaign it is revealed that his mother has been involved for many years in a conspiracy which began with the Congressional Medal of Honor and ends with an assassination attempt on the president-elect and, ultimately, the deaths of both Raymond and Eleanor.
Invoked with Ruprecht in Dirty Rotten Scoundrels. Lawrence has been posing as a prince to con money out of rich ladies. Whenever an heiress gets too close to his con, he gets Freddie to play a mentally disturbed brother to scare her off.
Lois McMaster Bujold's Vorkosigan Saga has the Vorbarra Imperial line. Thanks to inbreeding and genetic damage caused by environmental factors, some of the Vorbarra rulers have been... problematic:
Mad Emperor Yuri killed off most of his own family and then got dismembered and scalped by his own nobles, led by his brother in law/cousin.
Yuri's brother in law/cousin/successor Ezar was a relativly sane Chessmaster, but was also ruthlessly amoral beyond belief. The man signed off on a pointlessly aggressive war he knew Barrayar would lose to topple his political enemies and kill his own son.
Ezar's son Serg was a twisted sadist who probably would have destroyed the Imperium if he'd been allowed to take the throne. Ezar killed him in a Uriah Gambit (too bad about the grunts).
Serg's son Gregor inherited the throne at age five when Ezar died, and, remarkably, grew up sane and stable thanks mostly to his adoptive parents, Aral Vorkosigan and Cordelia Naismith Vorkosigan. But Gregor became so paranoid about the genetic insanity in his family line that he refused to consider marrying anyone even distantly related to him. Since that equated to all the nobility on the planet, there was no clear line of succession, and Gregor's death would have caused a massive and probably final civil war...this posed a bit of a problem.
Fortunately Barrayar has recently gotten a handle on genetic engineering, eased up on the social stratification, and annexed another planet with its own unrelated set of merchant nobility (one of whom Gregor eventually married), so that nasty strain of nutjobbus maximus is likely to be cleansed from the line in the future. Much to the relief of Gregor, Aral, Cordelia, and every planet anywhere near Barrayar.
Cursed royalty also appears in Bujold's Chalion books (she seems to like the trope). The main curse of madness/misfortune/sterility/whatever would make things most difficult in The Curse of Chalion is particularly nasty, in that it automatically spreads to anyone who marries into the family, making it completely impossible to eradicate without, as it turns out, direct intervention from the gods. The unlucky king Orico tried to short-circuit it by getting his wife, Sara, pregnant by his chancellor, because any child of theirs would not be part of the cursed royal bloodline. It didn't work because Sara was barren and also the chancellor was evil and his brother was an evil whackjob, but one gets the impression it was a clingy curse that would have come down on whoever inherited the throne, as well anyone around them who could potentially have finagled a way out. Thus, it took a miracle in the end.
And then there's that strange familial wolf-madness thing in The Hallowed Hunt, too. Revealed not to be madness, but an ancient shamanistic tradition that creates powerful animal spirits linked to certain rulers; the protagonist's dad just had the bad luck to pick a sacrificial animal that was rabid and bit him before it died, and the poor protagonist wound up convinced his own wolf-spirit would do the same to him.
Present in David Eddings' Belgariad, in the form of the royal line of Cthol Murgos, the Urgas family, with its hereditary insanity. In sequel series The Mallorean, the eventual successor to the throne is more or less sane, which makes sense, given that he's not actually the son of the crazy late king, but instead the product of a brief affair between one of the king's wives and a foreign diplomat. That's one effective way to get the crazy genes out of the royal line.
Made even more effective by the traditional method of ensuring easy succession: whoever gets the throne has every other potential claimant assassinated. Legally. Maybe the Murgos have had problems like this before...
Truth in Television — the early Ottoman Empire tried to cut down on wars of succession by having all male relatives of a newly crowned sultan put to death. Predictably this only increased the number of succession wars, as every potential claimant to the throne knew that upon the death of the old sultan he had to either win the crown or die. Urgit's quote of "It was either the throne or the block." in King of the Murgos is drawn straight from history.
The various Tolnedran imperial dynasties tended towards this as well. Typically the first few emperors of a dynasty would be clever, competent men, but after several generations of inbreeding the line eventually devolved into rulers who were insane, imbeciles, or both. And then subverted by the Borunes, who by their contractual obligations have to marry Dryads. Introducing exogamy into the family line every generation must help. Of course, female members of the Borune family are Dryads also (and exclusively Dryads, there's no such thing as a female half-Dryad)...
In Teresa Edgerton's Celydonn trilogy, it is revealed in The Grail and the Ring that Mochdreff has been politically unstable for centuries largely due to the land having been cursed due to the sins of its last ruling prince. He committed an action so terrible that every single member of his family changed their names and refused to take up the sovereignty - although only people like Dame Ceinwen remember even that much of the story, and nobody remembers the specifics. Ever since, there have been Lords of Mochdreff rather than rulers styling themselves princes, until finally, due to the lack of a clear heir to the previous Lord, Prince Tryffin was appointed Royal Governor and took it upon himself to try to clean up the matter once and for all by getting to the bottom of the curse.
In the Sword of Truth series, the Rahl family line, for several generations, have been warmongering psychopaths. The protagonist is, depending on the reader, either an exception, or adhering to the rule.
Everworld provides a variant: due to the royal tradition of Brother-Sister Incest, the last twelve Pharaohs of Everworld Egypt have all been mentally disabled and unable to rule in anything but name. This, coupled with the fact that the Egyptian gods are basically so obsessed with ritual that they've become willingly comatose, made the country weak and unstable enough for the Amazons to take over.
In P.C. Hodgell's Chronicles of the Kencyrath, the house of Knorth, from which the ruling Highlords come, has what appears to be an inherited tendency toward madness. Torisen, the current Highlord, is terrified of what lurks in his bloodlines, and of becoming like his father and grandfather. Inbreeding and deliberate breeding for Shanir (magical) traits is probably responsible.
Redwall's Marlfoxes. The mother Silth is a raving maniac, her youngest is a sneak who deliberately feeds her mother's paranoia in order to weasel (or fox?) her way into power, and the oldest six offspring are just plain nasty to various degrees.
The Kingdom of Delain, in The Eyes of The Dragon by Stephen King, suffers from this problem every now and again. Particular mention is made of Mad King Alain, who was truly a raving and unstable lunatic but did his people the favor of dying quickly — he decided to go outside and play games on the lawn during a raging thunderstorm (lunatic, remember?) and got struck by lightning.
The page quote comes from A Song of Ice and Fire, in which the royal Targaryen line is blessed with greatness as much as it is cursed with madness due to centuries of inbreeding. It started with the first Targaryen king, who was a great man but unfortunately married and had children with both of his sisters (a family tradition, his parents were brother and sister too); from there on out it's been a crapshoot. The line has produced many able warriors, statesmen, and scholars as well as a rogue's gallery of tyrants and psychopaths. Some Targaryens begin quite noble and lose their grip on sanity as they age, such as King Aerys II— by the end of his reign, he was known as King Aerys the Mad, and in the end his excesses sparked a revolt that toppled the dynasty. Daenerys, the only POV character with Targaryen blood (so far as we know) is somewhat more sane than her brother and father but not hugely so. The books give us only one normal Targaryen-Maester Aemon, as even the much liked Rhaegar was bipolar and had delusions of grandeur.
The Lannisters seem to be heading the inbred-madness route too: King Joffrey and his siblings Myrcella and Tommen are the product of Brother-Sister Incest between Queen Cersei (married to King Robert, whom she hates) and her twin brother Jaime. Jaime and Cersei's parents were first cousins. Cersei is a paranoid schemer who eventually engineers her own downfall, and Joffrey, her son was sadistic and unstable and had to be put down by Littlefinger and the Tyrells. Hopefully averted with Prince Tommen and Princess Myrcella, who are both perfectly sweet children... for now.
Interestingly, the Targaryen insanity problem seems to be entirely a question of the incest, as the Baratheons — a line founded by the bastard of an early Targaryen and in the modern generation descended from a Targaryen less than 60 years ago — all seem sane enough. Granted, Robert turned into a lazy Adipose Rex and Renly is a moron, but that's not insanity.
The Targaryen madness is, in truth, highly overrated. Aegon I-Perfectly normal, his son Aenys I-Fairly normal, his half-brother Maegor I-Psychopathic murder, his nephew Jaehaerys I-Best king Westeros ever had, his grandson Viserys I-Aside from not clearing up his line of succession quite normal, his son Aegon II-Paranoid, his nephew Aegon III-Depression, but stemming from watching his mother be devoured by a dragon rather than genetics, his son Daeron I-Became king far too young but not actually crazy, his brother Baelor I-Religious fanatic but nonetheless loved, his cousin Viserys II-Kept his predecessors from imploding the realm behind them, his son Aegon IV-Obese, corrupt, and horribly ineffectual, his son Daeron II-Second best king Westeros ever had, his son Aerys I-Obsessed with books to the exclusion of all else, his brother Maekar I-As stern, harsh, and unyielding as his great-great-grandson Stannis and even more unpopular but again not crazy, his son Aegon V-Sane aside from maybe the Summerhall incident, his son Jaehaerys II-Often considered weak due to his poor health and short reign but thought well of by anyone who actually knew him, and his son Aerys II-Psychopathic pyromaniac. That's only three crazy kings, two borderline crazy kings, and one horribly incompetent one out of the seventeen monarchs. If you want to count the Targaryens who never became kings, only Rhaegal (Brother of Aerys I and Maekar) and Aerion (Brother of Aegon V) were crazy.
The Targaryen madness really seems to be prominent in the more recent generations. Aerys II was the craziest of the lot, both of his sons had problems, his daughter isn't that much better and his possible surviving grandson also has issues (which actually convinces Tyrion that he really is a Targaryen), although his other grandson is fine other than being pretty emo at times.
In the Inheritance Cycle, there once was a King by the name of Palancar who tried to wage war numerous times with the Elves, even though every invasion was a hopeless crusade. Eventually his nobles rebelled against him to end the madness and had him exiled into a valley that later inherited his name. The Protagonist and his cousin, and the village they grew up in, descended from Palancar. Paolini so far has acted like that's at least somewhat of a good thing. Then again, he's also trying to convince us that the one who's really Royally Screwed Up is Galby.
Fiona Patton's Branion series is set in a fantasy Britain where the gods take an active interest in their followers. The royal family, whose head is called the Aristok, is literally touched by the gods — the sovereign is the avatar of the Living Flame, a deity/demon/primordial critter which is a sort of symbiotic parasite. This makes the Aristok something of a cross between a hereditary Christ-figure and the real British system of the monarch being head of the church. Not only does the Aristok have divine right, she can prove it. Unfortunately, being the physical sacred vessel-on-earth of a fire god is bad for your health. Out of forty-one monarchs, sixteen have died young, been assassinated, or committed suicide, and many of the rest went insane. Three even converted to a completely different faith, which made for real cognitive dissonance among their followers as well as themselves. Whether this system is a blessing or a curse on the royal family is clearly up in the air.
The first two of Mervyn Peake's Gormenghast books are, among other things, a long examination of this trope - the Groan lineage and their staff are a bunch of depressed lunatics, their spirits both crushed and perversely sustained by the castle and its ancient, messed-up rituals. They are a sympathetic bunch though - the melancholy and bookish Lord Sepulchrave and his unloved, cloudcuckoolander daughter Fuchsia must surely be among the most tragic literary woobies of the last century.
Historically, the rulers of Ankh-Morpork have tended to be raving psychopaths. This may have been somewhat genetic while the city was a monarchy and rulers' marriages were arranged to maintain the royal blood and survival tended to favor those homicidal enough to keep ahead of the Deadly Decadent Court. However, even after the kings were overthrown and replaced by the non-hereditary Patricians, the stress of the job and the absolute power that came with it turned pretty much everyone who held it some flavor of barking mad. The last two Patricians were known as "Mad Lord Snapcase" and "Homicidal Lord Winder". By a stroke of incredible good fortune the current Patrician, Lord Havelock Vetinari, is not only sane but an utter Magnificent Bastard and as insurance, the rightful heir to the line of kings is also hanging around the city, and he's also sane-ish, as he was raised by commoners...albeit commoner dwarfs.
The old noble houses of Ankh-Morpork, from which the Patricians are usually chosen, certainly haven't done themselves any favors through repeated intermarriage but, as with the royal line, this is implied to have contributed less to their overall unpleasantness than the self-selection towards dimwitted murder-happy prats (because of the military service requirement) and the self-absorption encouraged by privilege.
Interesting Times zigzags this with the Agatean Emperor. While his insanity is suggested to have a dash of genetic inbreeding behind it, the book hints (yet again) that intentionally breeding for paranoia and psychopathy, and then not telling the offspring why cruelty is bad, may have actually played a larger role than how closely related his parents were. Lord Hong, the real Big Bad, subverts the trope entirely by simply being a self-made Magnificent Bastard without any of Vetinari's redeeming scruples.
Played utterly straight when one book describes the lineage of kings in other Discworldian cities, and cites the last King of Quirm as having been so inbred he repeatedly tried to mate with himself.
In Jack Vance's Lyonesse trilogy, the king of South Ulfland's single son, Prince Quilcy, is feeble-minded and spends his days playing with fanciful doll-houses.
In the Tortall books by Tamora Pierce, the Copper Isles royalty tend to have madness crop up now and then, including one Princess Josiane. A character phrases it thus: "There's bad blood in the Copper Isles kings. They birth a mad one every generation. Josiane's uncle is locked in a tower somewhere. It comes from being an island kingdom- too much inbreeding." It turns out in further novels that it may not be just one per generation...
Two per generation, as of the Trickster books. The old king who dies and prompts the Succession Crisis and his brother who was mentioned as locked in a tower somewhere, and Josiane and Imajane among the old king's kids.
The Jimajen line might also have bits of this, though we only see two members: Rubinyan, whose only major flaws are an overdeveloped sense of honor and an inability to control his insane wife; and Bronau, who is extremely egotistical and ambitious without much common sense to go with it. Big brother is also ambitious as hell, but much more sensible...
Emperor Ozorne of Carthak and that cousin of the Tusaine line who starts the Tusaine-Tortall war in the second Song of the Lioness book both count.
Duke Roger, nephew to King Roald in the Song of the Lioness quartet, wasn't insane to begin with, but coming back from the dead (or not, precisely, if you believe him) certainly screwed with his head.
In the Tamír Trilogy (The Bone Doll's Twin, Hidden Warrior, and The Oracle's Queen) hereditary madness has hit the royal line. What makes this particularly dangerous is that the country's god has declared that only women of that bloodline can become ruler... or else. At the end... the sanest remaining member of the royal line takes the throne and the madness that caused the whole situation is just never mentioned again, since the epilogue indicates that there were no problems for centuries afterwards.
To be fair, it could be that Tobin/Tamír just took after Daddy more than Mommy in that regard. The royal madness seems to have begun with Agnalain II. Let's not forget all the ways in which Korin, pretty messed-up in his own right, was being manipulated and jerked around by Niryn. And finally, Tamír ends up marrying the indubitably sane Kirothius, who's not a noble and may therefore be just what the royal family needed.
Averted in the Honor Harrington series with the Star Kingdom of Manticore. Aside from being a constitutional monarchy, which limits the potential damage, Manticoran monarchs and heirs apparent are specifically prohibited from marrying members of the aristocracy. Aside from the "keeping in touch with the common folk" goal, it also removes the problems of inbreeding.
He swiped this from E. E. “Doc” Smith's Family d'Alembert series; under the Stanley Doctrine, nobility could marry commoners, but royalty was required to marry a commoner. While this helped, this was not totally successful in keeping loonies from the Imperial Throne (granted, the case of Empress "Mad Stephanie" could have been situational rather than genetic.)
In the case of the Stanley Dynasty, it may be as much cultural as genetic. At one point the competent, sane, and decent Emperor William (who is definitely an exception to the run of his ancestors) makes a joke about his and his wife's decision to abdicate at his age 70, so their daughter Edna "won't have to kill us." Edna is horrified by the joke, but her father points out that if he'd been more grasping and determined to hang on to power forever, decent Edna might have turned out different too, because, as he notes, 'like begets like'.
The author even points out that if the Monarch is really bonkers, Impeachment is in the Constitution, with Parliament choosing the new Monarch from any person in the Kingdom.
And before being added to the official line of succession, the Monarch's offspring have to pass a psychological and intelligence evaluation.
The Andermani Emperors on the other hand are competent but sometimes strange: the first emperor thought he was a reincarnation of Friedrich der Grosse (Frederick the Great of Prussia). Another was dethroned when he not only talked to his prize rose bush but also tried to make it chancellor.
And he was deposed by his own sister, who, while generally considered the best Andermani Emperor ever, had to legally declare herself a man, due to their Salic Law succession. May not have been the best decision for her own mental state.
It's hinted the reason why they are so successful is that the insanity and the genius go together. After all founding a New Prussian Empire on a Chinese colony world and making it into a regional power does sound pretty nuts. The first Emperor was a rich space pirate who saved the colony from starvation.
Actually, while she's generally sane, Queen Elizabeth III does have her own set of issues. Her temper, for instance, is usually described as "volcanic".
The Civil Government of the planet Bellevue in The General Series is Royally Screwed Up in that both the current governor and his acknowledged heir are borderline clinical paranoids, and becoming less borderline all the time...
Though, considering the political climate in the Gubierno Civil, the line between clinical paranoia and sane, reasonable social caution is slim indeed. The only reasons the POV character isn't a threat to the throne are his incorruptible idealism and his sure knowledge of the disaster that will ensue if he tries to take the throne for himself. Also, while the Cleretts my or my not be insane, they are also competent, if perhaps not excellent, political and military leaders.
A better example would be Settler Ali ibn'Jamal of the Colony, who is just an old-school psychopath.
The pureblooded Ancient and Most Noble House of Black from Harry Potter has a long history of insanity and inbreeding. Most pureblood wizarding families (with a few exceptions, like the Weasleys, who aren't hostile to mixed or Muggle-born in-laws) have gone this direction in recent decades, as the limited gene pool means they're all increasingly interrelated. Arthur Weasley says at one point that purebloods by the current definition will probably die out within a couple generations, as so many of them are already first cousins and within the incest taboo. Ironically, the Death Eaters probably managed to hasten this extinction quite a bit since many purebloods died in the fighting or were consigned to Azkaban afterwards.
The Gaunts are even more messed up and inbred than most purebloods, since Dumbledore mentions that they were the only remaining descendants of Salazar Slytherin and Marvolo was obsessed with the inferiority of other families. Ironically, the worst member of the family is conceived when they finally manage to get some new blood: Lord Voldemort, the son of Merope Gaunt and a Muggle.
In The Silmarillion, the first king of the Noldor, Finwë, is a good man, but after his death, the crown goes to his eldest son Fëanor, who was very paranoid before, and became completely crazy (if still very charismatic) when his father was murdered. Once Fëanor is also dead, his son Maedhros should become king...but he averts this trope: knowing how dangerous the Oath he and his brothers have sworn is, he abdicates and lets his uncle rule. It's a wise move: the sons of Fëanor do commit some horrendous acts, and the two eldest eventually become insane, but at least they only rule a fraction of the Noldor.
The royal family of Númenor also develop into this, as they become more and more jealous of the Elves for their long life, and determined to find a way to live forever.
The Bible is chock full of lousy or downright ax crazy evil kings of Israel who choose to snub the God who saved their ancestors from Egypt, so much so that the good kings are the exception.
And even the good kings still tend to be royally screwed up. Witness David, whose punishment for committing adultery with Bathsheba and having her husband killed was that his first son by her fell ill and died a week later, and the rest of the sons started killing each other for various reasons. David ultimately appoints Solomon as his successor, and even then the succession crisis doesn't end. Solomon was also messed up in his own right on account of his harem of foreign wives.
Averted in the Heralds of Valdemar series, in part because Valdemaran law forbids a monarch (or, presumably, heir) from marrying anyone within two degrees of kinship. And the newly-crowned Selenay plays it to the hilt to keep her councilors from forcing her into marriage, too. The requirement that all monarchs must be Heralds is also very important — there's nothing saying a Herald can't be a bit nuts (Hi, Lavan and Vanyel!), but at least it's the type of nuts that doesn't result in the abuses seen on the rest of this page.
King Rodric IV in The Riftwar Cycle. Hated and abused by his father for being a sickly runt, he proved to be as sick in mind as he was in body. Apart from using openly about how his power would allow him to randomly pick out random people and have them executed for no reason other than he wished to see them die, he squandred much of the tax revenues of The Kingdom Of The Isles on a series of aesthetic public works programs designed merely to make the city of Rillanon look prettier. Worse still, he denied vital military aid to the Western half of his Kingdom, fearing that the soldiers would be used to build an army against him, which helped to drag the first Riftwar out for the better part of a decade.
Though not quite royalty, the Usher family in Edgar Allan Poe 's Fall of the House of Usher fits. An illness that causes madness runs in the family, and it's implied to be due to inbreeding.
A rather desperate attempt to prevent this touched off the entire plot of A.L. Phillips's The Quest of the Unaligned. The royal house of the realm of Caederan, instead of being tied to one of the four elemental magics, are tied to all of them as the result of an ancient magical bargain binding the King and Queen to Caederan itself. Unfortunately, this means that if the King and Queen favor one element over the others, this will throw the land itself out of whack. A few decades before the story started, King Kethel and Queen Tathilya became increasingly infatuated with the power of air, which also had the side effect of causing them to become increasingly flighty and absent-minded. As the Balance fell further and further towards wind, the country was wracked by droughts, tornadoes, cyclones, and other catastrophes. When Queen Tathilya became pregnant, the nobles realized that if the new prince was raised in the royal court, he to would become infatuated with air magic. The nobles launched a desperate bid to separate Prince Alaric from the ruahk-controlled Court, and thus begins the plot.
In Babylon 5, the nephew of Emperor Turhan, Cartagia, became the Emperor of the Centauri Republic after his uncle's death. Emperor Cartagia was as bad as any fiction-version of Caligula, and apparently modeled after him.
Interestingly, the position of Emperor does not seem to be connected to any individual line for more than a few generations; it appears to be totally normal for the Centauri nobles to hand the throne to a new House even though the old imperial house still exists (Londo rules as Mollari II, as one of his ancestors had held the throne once). This presumably means that Cartagia's madness was a relatively rare occurrence in his House—rather as the Julio-Claudians only had two Emperors who could be called insane (Caligula and Nero), only one of whom (Caligula) was totally bonkers (Nero, while an awful ruler, was not totally incompetent).
One episode of Doctor Who strongly implies that eventually the British Royal family might become werewolves.
But merely because Queen Victoria had been scratched by the werewolf at Torchwood manor and infected by an alien blood parasite that used human bodies as hosts.
However at the time Victoria had already had all her children, so that idea has problems.
Referenced in the Red Dwarf episode Rimmerworld. Kryten explains the problem of having an entire society descended from Rimmer and his clones by drawing comparisons with European monarchies of the 19th and 20th centuries. The actual leader of the planet seems to be an example himself.
A lot of tension surrounding the Succession Crisis in Merlin is based on this trope. It's revealed in series three that the Pendragon men have a history of mental illness, one which King Uther ultimately succumbs to after his arguably Axe Crazy illegitimate daughter betrays him and takes the throne.
Game of Thrones implies this is caused by inbreeding in the Targaryen and Lannister family lines, in as little as one generation. The crazy rate is implied to be around 50% (as per the page quote), regardless of how long the inbreeding has been practiced.
Technically the Lannister case isn't a single generation; Jaime and Cersei are first generation sibling incest, but their parents were first cousins.
From BattleTech, the Liao dynasty of the Capellan Confederation seemed to produce only two kinds of rulers. Batshit insane and batshit evil. The non-insane non-sociopath Liaos tended to defect to other nations, neatly removing any descent chance of cleaning the batshit out of that genepool for a long time. Fortunately for the Capellan Confederation, the batshit evil variety tends to serve the needs of the populace; they're seen as evil by other nations that they're screwing over.
Also, the Steiner dynasty of the Lyran Commonwealth has a genetic predisposition to a few psychological conditions. They never get to the truly batshit level of insanity, but they have been known to do substantial damage to their nation.
It's applied to each of the five major factions at some point. Perhaps the most notorious version in-universe is Jinjiro Kurita, who ordered his troops to kill 52 million people on a world where his father was killed by a sniper.
Ravenloft had Legacy of the Blood, describing the relatives of the various Big Bads of the domain of dread, as well as options for PCs to play relatives of them.
The House of Naelax, rulers of the Great Kingdom of Aerdy in the Greyhawk setting, were commonly viewed as being possessed by demons. This article, although written by a fan for his own campaign, is nonetheless a good summary of what the Ivid Overkings were like.
Warhammer is fond of this trope. During the most decadent period of the Empire's history, it's implied that inbreeding reached epic proportions and led to actual mutations among the nobility. They seem to have straightened things out for the most part by the "present day" though. Thank Sigmar for the witch hunters, eh?
Inverted in the case of Bretonnia though, where it's the peasants who are inbred and deformed. Some recent anthropological research suggests this may be Truth in, er, roleplaying games.
Warhammer 40,000 has numerous examples of hereditary planetary rulers who follow this trope - though Inquisitor Vail would point out that this doesn't happen quite as often as the stereotypes would have you believe. In worst case scenarios (such as Osric the Loopy, mentioned in passing in The Traitor's Hand), the Officio Assassinorum can be dispatched to "tidy up" matters.
...actually, that's a lie. In worst case scenarios, the Royally Screwed-Up ruler is a heavily mutated Chaos-worshiper who unleashes the Legions of Hell on the planet they're supposed to be governing. In those scenarios, stronger steps are taken.
Of course, the Emperor was probably the sanest person in the galaxy and up until the Heresy the majority of his sons were perfectly sane as well. (The exceptions being Kurze, a murderous sociopath; Angron, a blood-crazed berserker; Lorgar, a raving zealot; and Alpharius, who had colossal delusions of grandeur.)
Somewhat justified in that the Silver Fangs had such an obsession with lineage that they refused to breed with any humans that weren't royal. So they managed to get most all the bad traits of just about everyone in the "Real Life" section below.
By the end of Electra, Chrysothemis is probably the only member of the royal family who hasn't tried to murder another member in retaliation for a previous murder.
Armed And Dangerous has an interesting case: a magical curse cast on the kingdom of Forge causes one king of the country to be a clever Evil Overlord, and his immediate successor to be a kind-hearted dimwit, and his successor again to be an Evil Overlord, and so on. In retrospect, it might have been a better idea to make the evil one the idiot.
Along with Pelagius there was his aunt, Potema the Wolf Queen, and Solitude is suggested to have endured more then its fair share of these kind of rulers in its history.
The Black-Briar family of Skyrim appear to be of the Environmental and Cultural types, being raised by a corrupt business owner in the Wretched Hive of Riften. Hemming is a stuck up brat, Sibbi is a sociopathic killer, and Ingun, the nicest of the bunch, has a strange affinity for alchemy, especially poisons. Close attention to dialogue also shows this to be In the Blood as well, as Hemming will refer to Sibbi and Ingun as both his siblings and his children, and looking at the game files shows that Maven is marked as Sibbi and Ingun's grandmother.
Fire Emblem. Good lord, Fire Emblem. Every freakin' game. Granted, no more than two games (except 1/11, 2 and 3) take place in any one continuity, but regardless, there is at least one mad ruler per game, or at the very least, mildly evil (Blazing Blade's King Desmond wasn't really mad, just a petty idiot—and Marquess Laus wanted to rule all of Lycia, but never actually did.) Well, okay, Radiant Dawn actually had a bunch of evil senators trying to usurp the empress of Begnion and an Evil Chancellor at the side of the new king of Daien...FE10 did have Naesala, but he turned out to be...compromised.
Fire Emblem Awakening goes one step further: not only is the main villain the result of a thousand years long program of eugenistic breeding among plegian royalty meant to create an avatar to a demonic genocidal city-sized dragon, but this time, s/he's the protagonist: Fire Emblem: by Nintendo, the company known for its colorful family friendly games
The Dresari family in the MechWarrior 4 series appears to suffer from this; it's doubly painful because the likable player character in the first game pulls a Face-Heel Turn and becomes The Caligula in one of the expansions. Per a previous example, this is not entirely uncommon in the BattleTech universe.
Weirdly enough, Word of Godretcons this saying that the latter incident mentioned above is in fact propaganda from the aforementioned Steiner ruling government, whose leader at the time was not above this or numerous other antics reaching to the Moral Event Horizon.
Zork - the entire Flathead dynasty, ruling or not.
Nearly every clan in Drow Tales could fit into this, but the Sharen are the most screwed up. Matricide, starting a civil war, and subjecting one's entire clan as well as any female summoner to demonic Tainting, is a good start for proving a case of mental imbalance.
The Sarghress clan apparently has a history of child abuse. Allegedly, Quain'ana ordered her soldiers to rape her own daughter Mel'arnach when Mel refused to bear an heir for the clan; in turn, according to a non-canon side story, Mel and Sil'lice raped their adopted sister Syphile, and Syphile once locked Ariel (who was physically about 5 years old at the time) and Fuzzy (Ariel's cat) in a cell with no bathroom for a week, and then killed Fuzzy in front of Ariel when Fuzzy bit her.
It's noteworthy that while Kharla is an adopted member of the clan, the native-born Vloz'ress apparently have a streak of madness a mile wide.
Kiel'ndia: So, what do you think of my home? Sucks, doesn't it? I wonder what was going through their heads when they built this place. "Let us create a monument to immortalize our madness, to be cherished forever by nuts worldwide. All loonies shall live here and despair... MWAHAHAHAHAHA!"
This problem is endemic in Girl Genius. Sparks, being creative geniuses with impulse control/prioritization issues, naturally respond to any intellectual problem or technological innovation with "ooh, shiny!" The powerful ones also tend toward considerable charisma and psychological instability. Throw in a lot of "manifest destiny" and "right to rule" noble sentiment, probably lifted from real-world history, and you get feuding warlord dynasties unleashing war machines and fearsome monsters upon one another constantly. Anti-Villain Baron Klaus Wulfenbach forged a Pax Wulfenbach of sorts, but there's still a fair amount of scheming and rebellion against the (perceived) Evil Overlord. Even the heroic Sparks, mainly heir to legendary heroes Agatha Heterodyne and her probable love-interest/only viable political rival, Klaus's son Gilgamesh Wulfenbach, are prone to manic episodes of creativity and occasional violence.
The House of Heterodyne, of which Agatha is the only known living member, deserves special mention. For generations, they were the most insane and dangerous maniacs that the world had ever known, and also some of the strongest Sparks. The previous generation, Agatha's father and uncle, are an exception, having used their brilliant insanity for good; but according to one observer, the people of her hometown would accept a crazy Heterodyne as legitimate:
Vole: De pipple of Mechanicsburg would not ekcept dot [killing Castle Heterodyne] as proof dot she iz a Heterodyne...not unless she danced nekked though de ruins vile trying to shoot down de moon—turned all de tourists into monsters—and den built a very dangerous fountain out of sausages.
Add in the fact that the Hetrodynes were the ones who created the Jagers (think WW1 Germans fused with Orkz and muppets) and they were plenty messed up too. Basically, the Hetrodynes bred right past crazy and back around to normal.
Princess Sara in 8-Bit Theater is smart, sexy, and sane enough to fully realize her father is a Cloud Cuckoo Lander with genocidal tendencies. Naturally, she doesn't hold much stock in hereditary rule. She's still a rude, shrewish sociopath, though, and engineered her own kidnapping.
How bad it is: No matter what horrible evils she unleashes on the populace when she comes into power, it will look like a golden age compared to the completely ruinous and unhinged chain of decisions King Steve makes every day, simply because she's not enough of an idiot to be capable of the same levels of casual destruction.
It's even worse than that: King Steve boasted a 52% approval rating. He got this by having pollsters ask which would they prefer: Having Steve as their king or taking a sword to the head. 48% of his subjects chose execution (And received it).
The Masters Royal Family of Chess Piece are said to be cursed. Luckily, it skipped a generation. Unluckily, the current Prince has seriously planned on taking over the world since he was four.
Though not actually royalty, whenever Achievement Hunter does a "King" episode in their Let's Play Minecraft series, if Ryan is king, he will flat-out go nuts. To wit, "King" Michael and "King" Geoff were quite simple in their events; Ryan built a Russian Roulette room and told the other guys to go in and test their luck.
Candy Kingdom Law, in Adventure Time, is "complicated," according to Princess Bubblegum. In the event that anything should happen to PB, who inherits the throne? Her maladjusted, overly-sensitive, socially awkward, most likely brain-damaged, insensitive, angry, sour-tempered, alienated son/science experiment gone wrong, Lemongrab. Arguably, Lemongrab is a pretty sympathetic example of this trope—he obviously has an... ahem, a delicate condition, but that doesn't stop everyone from despising his guts for being a Jerk Ass most of the time and sending everyone to the dungeon for a million years. But why stop there? When the princess realizes her errors, she must create a second heir. This time, she brings her own DNA into the equation in an attempt to stabilize the formula. Enter the giant, pink, infantile being Goliad. Princess Bubblegum wants to educate her in all the ways of royalty and leadership and Finn and Jake are tasked to teach her. However, Goliad's "mondo mama brains" have an extreme imprint. Just seeing Jake yell at a group of preschoolers in order to keep them quiet send her into an authoritarian state in which everyone is forced to follow her rules. Then Goliad pulls out her Third Eye and begins to control the citizens of the Candy Kingdom with mind control and telekinesis. If not for Stormo, a being created by Finn's DNA, the kingdom would have fallen.
Surely the Candy Kingdom can't have all the fun. There's also the Nightosphere and the Vampire Kingdom, controlled by the Abadeers, respectively Hunson and Marceline. Hunson, as the Lord of Chaos, controls the strange tortures that flood the Nightosphere. Mutilation, immolation, bananas coming out of orifices; all done in pure glee. His daughter, Marceline the Vampire Queen, is seen less as a ruler and more a fickle trickster. If she has royal obligations, she completely ignores them.
At one point, Marceline was tricked into becoming the Lord of Chaos by her own father. This made her even more disturbing as she would line up those in the Nightosphere and hand out punishments on whim, giving choices like "pain, pain, or more pain" and asking if someone wanted abs (which she placed on the person's head).
The entire Fire Kingdom court is evil. This includes Finn's new Love Interest Flame Princess, though her father admits that love could turn her chaotic neutral (at the cost of an experience penalty for going against alignment). She's also Cute and Psycho with a Hair-Trigger Temper, and if her emotions go out of control she can destroy the world. Her father is also a little too eager to make sure his daughter stays evil.
The line of the Fire Lords in Avatar: The Last Airbender has... issues. Specifically, a tendency toward being sociopathic and homicidal on both a personal and national level. Again, if there's hope for stopping the ruling lunatics, it seems likely to come from the branches of the tree that didn't get hit with the genocidal batshit crazy stick - disgraced traitor Iroh, or screwed-up-but-trying-to-improve disgraced traitor Zuko. For the most part the Fire Lords seem to have avoided taking out their issues on their own people, so their own common folks seem to be reasonably pleased with their rulers. It's just everyone else on the planet who's rightfully terrified. The problem isn't likely to go away until the planetary balanced is fixed; it seems to be spiritual in nature. (One ancestor went power-mad three generations back, and his successors have continued his policies. And why not, as they seem to be working fine - as long as you're Fire Nation, that is.)
In Sweden, the Vasa line of rulers was known for this attribute. Gustav Vasa was a competent tyrant who united Sweden and arranged Reformation in Sweden in order to plunder the riches of the Church to fund his endless and pointless wars; his first son Erik XIV was absolutely insane (killing servants who wore colorful clothes) which may have been because arsenic was added to his pea soup as a food colouring, the middle son Johan III was a brutal warlord who turned into a neurotic depressive; the youngest son Karl IX was a paranoid madman who ordered several massacres and a religious zealot who extirpated Catholicism off Sweden and insisted instilling Calvinism. The son of Karl IX, Gustav II Adolf was definitely competent and is still a national hero in Sweden, but also arguably a scary and megalomaniac Blood Knight. His daughter Christina was actually quite competent, if a littleeccentric.
Let's not forget Prince Magnus (brother of Erik, Johan, and Charles (Karl)), who was too mentally unhinged to even participate in his relatives' infighting. Interestingly, letters show that the other three seemed to actually have cared for the insane Magnus a lot - despite showing murderous hatred towards each other in other circumstances. On the other hand, he was never a threat to them...
There's also Sigismund Vasa, who somehow was elected king of Poland-Lithuania. While relatively mentally stable, he was stubborn to a fault, refusing to see Poland-Lithuania as anything else than a tool to get him the Swedish Crown, even though Sweden didn't want him, as he was Catholic. His reign led to nearly constant war between Poland-Lithuania and Sweden and Russia for the next 100 years, which ruined the country and led to its eventual partition.
The Vasa line survived far longer in Poland after it had become extinct in Sweden. The Vasa kings of Poland represented themselves as the legitimate claimants of the Swedish crown.
Henry VI had some kind of mental illness which left him near-catatonic for long periods. It may have been inherited from the French royals; Henry's French grandfather, Charles VI, was also mentally ill, sometimes claiming to be made of glass. Charles's madness led to a civil war, and a English invasion - Agincourt and Joan of Arc; Henry's madness led to the War of the Roses - two wars, one of which effectively purged the English royal line of madness by almost exterminating it. This shows that occasionally Real Life can be more sensible than fiction: most medieval kings had to be competent, or they got removed.
Charles VI's madness was triggered by two traumatic events, the first possibly induced by sunstroke, the second by the notorious "ball of the burning men", in which Charles himself came close to being burnt alive. And while Britons like to point the finger at Charles VI as being to blame for Henry VI's madness, it should be noted that his French offspring was not crazy, but actually seemed to have been pretty smart and rational by the standards of their day, in particular his grandson Louis XI, known as the Universal Spider, and his Louis's daughter Anne de Beaujeu, who very competently ruled France and won another civil war during the minority of her younger brother Charles VIII.
Much later there was George III, of the "talking-to-a-tree-because-he-thought-it-was-the-king-of-Prussia" style of crazy, but that was an isolated incident and might even have been made up by his son, George IV, who had to rule as regent for years while they waited for him to die. (These are the Georges that turn up in the Blackadder season 3 finale, incidentally.) The king Georges had an unspoken family tradition of having bad relationships with their oldest sons and IV is known to have cruelly parodied III's condition in front of his friends in London clubs. IV himself was more a case of 'just about sane enough' than completely well-balanced; fortunately power was sufficiently shared with Parliament by this time that a sane-ish monarch was good enough.
George III only developed madness in later life - earlier in life he was charming, handsome and reasonably well-adjusted. Recent theories suggest that this may have been due to a genetic condition called porphyria that is hereditary, it just tends to skip several generations without manifesting. May have entered the British Royal Family from the Scottish line James I and VI who may have inherited it from his mother Mary, Queen of Scots. Mary, James and George are just the only reigning monarchs to suffer, all the other possible instances just cropped up in branch lines.
Skeptics of the theory that George III inherited porphyria from either Mary and James have pointed out that that there is as yet no evidence of porphyria in the intervening generations of Elizabeth, Queen of Bohemia; Sophia, Electress of Hanover; George I; George II; and Frederick, Prince of Wales. Certainly neither Mary nor James displayed any signs of madness. In any case, George III's doctors tried to cure his madness by giving him mercury ("It works on syphilis, maybe it'll cure this too..."), which obviously didn't help.
Another doubt is cast on the porphyria idea is that one of its defining features is bluish urine... except George was also being given a herbal sedative which causes blueish urine as one of its known side-effects. There's also the curious fact that he 'got better' for a few years between two attacks of his madness; the descriptions allegedly make the first sound like a long hypermanic episode, and the second (when in his seventies) sound more like dementia than the 'madness' it was described as.
Henry VIII started out as a good king, but became more and more cruel and egotistical as he got older (as many of his wives discovered). Theories differ on why, but illness, a sports career ending injury, and the lack of a backup male heir have all been put forward as helping him along the path from arrogant and short-tempered to paranoid megalomaniac.
The sanity of his daughter Queen Mary I (not only Britain's first ruling queennote Unless you count Matilda aka Empress Maud. but the original Bloody Mary) is a question historians have never settled. Her campaign to re-establish the Catholic church led to an unusually large number of brutal executions in her six-year reign and brought the country to the point of outright rebellion. Of course, the English Reformation had started as a way for her father to divorce and mistreat her mother, who was then dying of cancer, and witnessing that had a profound effect on Mary. She also may have had a hysterical pregnancy: when she married at 38 she was understandably desperate to produce an heir, as the only other surviving Tudor — her younger half-sister Elizabeth — was of questionable legitimacy and very Protestant. Mary was observed by her courtiers to show every sign of pregnancy… but a year went by and no child was born. For centuries it was assumed to be extreme wishful thinking, but the fact that she died soon after suggests it may have been cancer or some other major physical illness.
The Habsburg dynasty of Spain, Austria, and the Low Countries claimed to be the rightful successors to the Roman Empire and the Holy Roman Empire. They were also excessively inbred even by the standards of late medieval European royalty, with a distinctive underbite known as the "Habsburg lip" that got more disfiguring in later generations and a tendency towards mental instability. The Habsburgs felt that not marrying fellow high royalty was beneath them and were also devout Catholics, meaning half of European royal families were off limits after the Reformation. Furthermore, the greatest remaining Catholic monarchy was France, the Hapsburgs' greatest rival (although they did sometimes accept marriage proposals). This meant that the Spanish and Austrian branches of the family kept trading marriage partners. Marrying into the Spanish House of Trastamara in the early 16th century had been a shrewd political move to expand the family's power but a poor choice for genetic health as the pre-Reconquista Spanish houses were already pretty inbred (the rest of Europe had basically written Spain off as permanent Moorish territory and had been unwilling to arrange marriages there for quite some time). Even by that low bar, the Trastamaras were noted for their history of oddballs, although the number of truly crazy family members may have been overstated by their enemies.
Larry Gonick's take on the Trastamaras was "I'm Pedro El Cruel! What can I do to you?" ...which was the king in question's real nickname, at least among his enemies. His other nickname, "Peter the Lawful", could be taken to refer to his skill as an administrator...or his penchant for beheadings. The actual founder of the Trastamara line was Pedro's sane illegitimate half-brother but that didn't keep the unfortunate recessives from popping up in subsequent generations.
Don Carlos, the rebellious son of Philip II, was insane to the point of being physically dangerous and would take swipes at passing servants with a knife.
Ferdinand II's favourite occupation was rolling around in the bin.
All Habsburg lines tended to lead back to "Juana la Loca", a.k.a. Queen Joanna the Mad of Aragon and Castille...over and over. It's debatable how mad she actually was and how much of that was genetic versus how much was induced by grief and mistreatment after the death of her husband Philip the Handsome. Witnesses who weren't paid by the rivals to her throne contested the incidents claimed as the most serious evidence of her insanity, such as repeatedly reopening her dead husband's casket. (Yes, her throne: technically she inherited it from her mother Isabella and was a queen in her own right, a fact that annoyed her father, husband, and son in turn, as they strove to prevent her from asserting her authority.) She probably did have some form of hereditary depression but her "madness" was worst when she was locked up in a nunnery by her own father Ferdinand. Her son Carlos I/Charles V had to be told to treat his poor mother better as a condition for election as Holy Roman Emperor. note Yes, this is that Charles V. The one who might have been ruler of all Europe (save England and France) had it not been for the emergence of Protestantism. That one.
Joanna was the older sister of Catherine of Aragon, first (and some would say only) wife of Henry VIII. Her daughter Mary Tudor wedded her first cousin once removed Philip II, but they never produced a child and Philip eventually remarried with his cousin Anne of Austria. Had the English Reformation never happened, the history of the Hapsburgs may have been quite different. On one hand, a Catholic England would have opened up the gene pool a bit for the Habsburgs. On the other hand, England still might not have provided enough genetic diversity if the Habsburgs kept marrying only Habsburgs (it would probably depend on whether the English Habsburgs continued the fine British tradition of having the Spare To The Throne inherit; the spare could then marry minor Italian and German Catholic nobles), and England might have suffered royal insanity to the same extent as the continent.
Emperor Ferdinand I of Austria wasn't the head-choppy kind of crazy but he was definitely epileptic and had a hydrocephalus. His (rather euphemistic) honorific was "the Kindly" or "the Benign" (Ferdinand der Gütige in German).note After his 1848 abdication, jokers sometimes turned this into Gütinand der Fertige: "Goodinand the Finished". The only direct and coherent command he ever gave during his reign was "I'm the Emperor, and I want dumplings!", upon being told that the apricots needed to make the kind he wanted were out of season. After being told by his chancellor Metternich that the people outside the palace were carrying out a revolution (in 1848), his answer reportedly was: "Yes, but are they allowed to do that?"note Incidentally, he's said to have said it in the Viennese dialect of Austrian German, which (for German-speakers) lends to the air of amiable cluelessness. Since he remained childless, his disabilities did not continue down the line. Not that the line would ever have become Emperors; after the aforementioned revolution, the government convinced him to abdicate in favor of his saner and (it turns out) hypercompetent nephew Franz Joseph (who would reign until 1916).
The Austrian Habsburgs were willing to accept Hungarian nobility as marriage partners, especially after the reorganization into a dual monarchy in 1867, but not nobility from any of the other constituent ethnic groups of the Austro-Hungarian Empire. This lead to a falling-out between Emperor Franz Joseph and his heir-apparent, Franz Ferdinand. Franz Ferdinand made the unthinkable faux pas of marrying a Czech countess, whom his relatives did not recognize as "real" nobility,note Well, real, but not up to Habsburg snuff: to marry into the Imperial House, one had to be a member of a "ruling house" of Europe, i.e. of the ruling family of a sovereign European state while all Czech nobility were Habsburg subjects. Most ruling houses followed a rule like this, but few adhered to it as closely as the Habsburgs (for contrast, the younger sons of English and British monarchs were encouraged to marry English/British noblewomen, and it seems that the British married their elder sons to royals mostly for the political advantages) and his children by her were ineligible for the throne.
Whether or not the Imperial Roman lines count is a matter of much discussion; certainly many of them were raving mad by our standards, particularly, well, Caligula. For an entertaining view on how insane things got, check out I, Claudius. How much of that was due to the Emperors being corrupted by absolute power, or due to environmental factors such as heavy metal poisoning (a sweeter wine is all well and good, but lead additives aren't going to be a health craze any time soon), or how many Emperors just seemed paranoid and vicious because that was the only way to survive as a Roman Emperor (only 29 out of 88 Byzantine Emperors died of natural causes, and at least a dozen were murdered by their own relatives), or how much of the crazy was made up by much later historians like Tacitus and Suetonius to serve as parables, is both debatable and debated. The Empire alternated between periods of military autocracy (where the army essentially chose the leader) and dynasties founded by relatively competent emperors that inevitably descended into murder and madness within a couple generations. A sitting emperor's smartest option was usually to adopt an heir of proven sanity and military aptitude.
Queen Victoria was a hemophilia carrier and passed the gene on to three of her children, from whence it spread to many other European royal houses, royally screwing them over. This included the Romanovs, whose last legitimate heir was doomed to die from this illness before maturity; that's why the faith-healer Rasputin The Mad Monk gained such sway over the Empress. The hemophilia gene became so prevalent among royals that many commoners thought that hemophilia was the genetic marker of royalty. Even the 1950s B-movie Queen of Blood decided that the extraterrestrial featured must be royalty solely because she was a hemophiliac. That is one impressive little allele.
Not that the Russian royal line needed much help from Queen Vicky. When you've got such kings as Ivan "the Great" (yes, he did tonnes; could also start a brutal war over not agreeing with anything any of his brothers liked), Ivan "the Terrible" (ye gads, the guy knew how to do crazy: and, ran through wives and children at an impressive rate), or even Peter "the Great" (who... very much had his moments of Cloudcuckoolander bonkers, despite the "Great") in your genetic back pockets, you've got problems. Even Catherine "the Great"'shusband was a little unfortunate to have her as a wife (even though they were both far more German than Russian). As she probably killed him. No matter what your surname was or who your supposed father was (there are a lot of questions at various points)... the same lesson can be repeatedly found in every Russian linage to hold that throne: don't trust family as they'll probably kill you. Likely on purpose.
This lead to the abolition of the 240-year-old Royally Screwed Up Nepalese monarchy in 2008. In 2001, Crown Prince Dipendra allegedly went Ax-Crazy and gunned down most of his relatives, including his father the King. By law, Dipendra was crowned Nepal's new king, despite being both accused of multiple murders and comatose from an allegedly self-inflicted gunshot wound. Dipendra quickly succumbed to his injuries and his uncle Gyanedra was crowned king despite being widely suspected of masterminding the royal family massacre. No matter whether Dipendra or Gyanedra had really done it, Nepal had made at least one mass murderer its king. When Gyanedra tried to abolish Parliament, the Nepalese decided they had had enough of this trope and voted to make peace with the Maoist rebels and declare a republic.
Ancient Egyptian pharaohs made something of a habit of marrying their eldest sisters, to the extent that archaeologists thought for awhile that claim to the throne might have passed through the oldest daughter of the late king because they couldn't understand why else so many princesses became their brothers' Great Royal Wives. Current theory says that it was in fact male-line inheritance and that many of the marriages were symbolic, meant to prevent princesses from marrying other men and establishing cadet branches of the royal family. HOWEVER the imperative to secure the bloodline (and a general Egyptian distaste for marrying non-Egyptians) meant that some of those sibling marriages were also not purely symbolic. The Ptolemaic dynasty (which was ethnically Greek but very quickly assimilated to pharonic tradition) in particular had a Moebius family tree and it kind of showed in some of the later Ptolemies, Berenices, and Cleopatras (historical accounts indicate that Cleopatra VII, the one everyone remembers, was unusually charming and savvy by her family's standards).
Ibn Khaldun's surviving work deals specifically with North African bedouins, but he established a more generally applicable generational succession for nomads and the civilizations they conquer: the first generation is rough and tribal and not quite civilized, the second (with luck) still understands what made his father strong but also has been brought up in the arms of city culture, the third begins to lose touch and grow soft, and after that discipline just fall apart until the next new dynasty rides in and replaces them. Rinse, repeat. Ibn Khaldun was one of the first historians to have a theory of history instead of just a straightforward record of what happened. The Islamic dynasties of Spain are an excellent example of this paradigm and We Have Become Complacent on this very wiki is a relative of his theories. While he mostly gave North African examples, you have to bear in mind that the Muqaddimah (which is where these theories come from) was actually the introduction to a much larger history of North Africa and the Mediterranean (muqaddimahjust means "introduction" in Arabic). With the possible exception of the Roman and Egyptian empires, pretty much every great empire of the Mediterranean region in the pre-modern era was founded by at best half-civilized conquerors who took over the established civilizations: the Akkadians, Assyrians, Hittites, Persians, Macedonians, Germans, Arabs, and Turks (to name only the most obvious examples) all did this. Even the Romans were pretty uncouth when they started taking over Italy; between their militarism, agrarianism, lack of high culture, and piety,note Rome was noted as the most pious city in Italy in that era the perception the Etruscans and peninsular Greeks had of the Romans was the Late Antiquity version of Flyover Country. And as for the Egyptians, they only expanded to become a true empire after their country was conquered by foreign barbarians (the "Hyksos", who were probably Canaanite shepherds), and they won that empire by adopting the barbarians' technology and tactics.
Ludwig II of Bavaria was considered by many to be mad, and deposed because of it (based on the "diagnosis" of Dr. Gudden, who wrote it without ever having met Ludwig in person), followed by his death the next day of either murder or suicide. For the most part his 'insanity' consisted of an obsession with building elaborate castles, opera music, and beautiful men, and he's more fondly remembered now.
After that, his brother and successor, King Otto, spent his entire "reign" institutionalised. (He'd already been declared mad and consigned to an asylum during Ludwig's reign.)
Some Bavarians like to claim the madness of (possibly) Ludwig and (definitely) Otto on their Prussian mother Marie, pointing at the case of Frederick William IV (uncle of Marie of Bavaria), however new evidence suggests that the Frederick William did not actually go mad but suffered from the effects of a stroke in his later years, which his Bavarian consort Queen Elisabeth tried to hide from the public.
Like Roman elites poisoned by their lead plumbing, research on remains of samurai children buried at a Japanese Buddhist temple showed sky-high lead levels believed to be from the lead-based white face powder used by upper-class women. Chronic heavy metal poisoning may have contributed to political instability and the decline of centuries-old shogun system in the latter half of the 19th century.
Some Biblical scholars believe King Saul may have been schizophrenic. Not only because of his jealous obsession with and multiple murder attempts against his eventual successor David, but also because he threw a spear at his own son Jonathan for merely asking why David had to die.
Emperor Taisho of Japan, due to a combination of inbreeding and meningitis, was known for bizarre behavior in his later years. During a parade, he reportedly hopped off the royal float and hugged a random trumpet player in the accompanying band.
Frederick William I of Prussia had porphyria, and also liked to carry a wooden cane, much to the regret of everyone around him- he chased his children around the palace with it and was known for randomly attacking commoners in the streets of Berlin.note Who naturally couldn't defend themselves, as attacking the king was treason, a crime worthy of execution. This seems to have had a negative effect in his relationship with his son, Frederick The Great, although the former's complete and utter contempt of Frederick surely didn't help.