Many characters in Yu-Gi-Oh! and GX fall into some Dark Side's trap, usually by trying to use an Artifact of Doom or something Powered by a Forsaken Child (i.e. Marik, Dartz, Akunadin, Saiou, Judai), sometimes with good intentions (Yami Yugi used the Seal of Orichalcos partly to save his Dark Magician Girl, Fubuki used his Superpowered Evil Side to try to save Ryo). They start confident they'll be able to control what they unleash, but inevitably, the power's/object's inherent darkness is controlling them.
Light can function the same way. A malignant celestial radiance known as the Light of Destruction tried to destroy the world via brainwashing cult in Season 2. Even after its defeat, it continued to be referenced thereafter as one of the most dangerous forces in existence.
Used straight and subverted in Bleach: gaining Hollow powers raises people's upper power (it is stated that shinigami have a maximum power they can attain) and allows them great spiritual growth. But it also risks giving into your Super-Powered Evil Side for a world-class Freak Out. The Vizard thoroughly beat their dark sides into submission in order to access the power safely, and teach others in their dilemma how to do the same; however, not all people who gain Hollow powers want to suppress The Dark Side.
Mahou Sensei Negima! has Negi learning Black Magic so that he'll be able to protect his students, and it's implied that there's a real risk of the power taking control.
It's also used more humorously, as Evangeline will attempt to "turn Negi to the dark side'' whenever the opportunity presents itself. It hasn't worked yet.
She touches this with almost everybody she comes into contact with, and has noticeably affected some of them (including Negi) — the catch is that this setting weighs heavily on the side of Dark Is Not Evil. Her version is more aesthetic choice and antisociality with Dark Side corruptive qualities than what the (other?) villains in the series possess.
Does appear, however, in a 'What If...?' in which the symbiote latches on to the Punisher after Spider-Man rids himself of it, and attempts to corrupt the Punisher into killing Spider-Man. Subverted, however, in that the Punisher — who is already a pretty dark character anyway — is having none of it, manages to gain control of the symbiote, and informs it that if it ever tries to make him hurt an innocent again, he'll kill them both.
In Marvel's World War Hulk: X-Men, Cain Marko succumbs to the lure of being an unstoppable force once again, and his own innate desire for destruction, which leads to his return as The Juggernaut of old by fully embracing the power's evil nature.
More recently, in Fear Itself, Colossus, in a desperate bid to save the X-Men from a jacked-up Juggernaut, absorbed the Cyttorak crystal into his own body, becoming the new Juggernaut. He is now struggling to hold on to his humanity and prevent Cyttorak from completely possessing him. Judging from some previews of Avengers Vs X-Men it's going to be a losing battle...
All of this turns out to have been his own sister Magick's scheme to make him realize that she isn't his innocent little sister anymore. By allowing him to be Juggernaut for a while (she could have freed him at any time with her own powers), she made Colossus experience firsthand how the Dark Side changes people.
The Shun Leep martial arts style from Gold Digger is so badass it has its own dark side for those who don't have the self control to master the higher techniques. It's not even a magic-related style! It just makes fighting so euphoric, and actively addictive, you stop caring about little things like 'whether your opponent lives'.
We must mention Jack Kirby's enduring creation Darkseid, the local incarnation of... well, The Dark Side.
Parallax, the living enbodiment of fear, who corrupted Hal Jordan into a maniac, and tried to do the same to Kyle Rayner.
Sinestro, who was corrupted- not by Parallax, but by trying to bring order to his home planet by creating a fascist rule. He later allied himself with Parallax to try bringing order to the entire universe.
The Red Lantern Corps, whose rings are powered by the wielder's own rage.
The Star Sapphires are powered by love, with much the same addictive effects.
The Orange Lanterns whose powers are derived from greed, and the Black Lanterns the "darkest" of the Lantern Corps in general, being dedicated to death and darkness, with all its members being undead.
Subverted and averted with the Orange and Black Lanterns respectively. The Orange Lanterns are nothing but energy constructs that take the form of Agent Orange's many victims. Then again, all of them were beings driven by greed in the first place, which is what drew Agent Orange to them, so it still sort of fits. Agent Orange himself is a prisoner of his own power, hopelessly addicted to it and unwilling to ever let go of the Orange Lantern Battery which contains Ophidian the Greed Entity despite the unending hunger it inflicts upon him. The Black Lanterns are nothing but corpses animated by the rings themselves which are all mere extensions of Necron's will. There's no trace of the people that used to be those bodies.
Anakin Skywalker/Darth Vader is by far the most famous Star Wars example, but there are plenty of others in the Expanded Universe. (See also under video games.) According to the source material, the Dark Side is powerfully addictive, since not only does it give you visibly stronger powers (at first), it also is a tremendous rush: it doesn't just feed off of negative emotions, it enhances them to some degree, and the more you use it the more susceptible you are to continued use, like an addict. There's plenty of evidence to show it can be resisted and fought off, but that's like asking a recovering heroin addict to hold onto some heroin for awhile: you ARE taking a chance that he will use it.
Same really applies to anything evil in the whole mythos this side of Morgoth, which seems to be the "root evil" of sorts. Which raises interesting questions about Ilúvatar's motives in creating him and not stopping him (it's difficult to believe he was truly deceived or powerless to), but that's neither here nor there. As stated, practically every other evil character, or character committing evil acts, is subject to the Dark Side Temptation. Sauron wasn't the only Maia he tempted, and a case could be made for many Humans and Elves throughout the story.
The reasons for his creation, and Eru allowing his corruption and not stopping him are actually given in the text, even while the explanation itself is serving to further the corruption:
Eru Ilúvatar: And thou, Melkor, shall see that no theme may be played which hath not its uttermost source in me, nor can any alter the music in my despite. For he that attempteth this shall prove but mine instrument in the devising of things more wonderful, which he himself hath not imagined.
We are not told that Morgoth was created as evil and given Tolkien's philosophical views it seems doubtful that he was intended to be so. The answer appears to lie in The Evils of Free Will.
Tolkien was a devout Catholic and it shows a lot in his mythos. Melkor really had no provocation to become evil, though, or to be overly proud; it just kind of happened ... actually the only reason he became prominent was because he interrupted and twisted the Song of Ilúvatar. Which may be a way of saying that no creation of Ilúvatar is perfect (the Valar, Morgoth's opponents, had plenty of flaws themselves).
Melkor was explicitly the most powerful of the Ainur, which would probably explain the pride. Exactly how powerful he was Tolkien was never quite able to pin down (ranging from being only slightly stronger than his brother Manwë to being as strong as all the other Valar combined, and usually somewhere in between), but he pretty clearly saw himself as the pinnacle of creation. In any event, he didn't actually rebel until the Music, when he decided he could do it better.
"No provocation to become evil?" Let's have a look at that again: He was described to be aloof from the others, animcable enough just not a team player. When everyone was creating the world, any works he presented were nixed, even though they were described as beautiful and masterful. When it was done, none of his effort had been accepted into creation, for no reason given, but the implication was that the other Valar simply felt that Loners Are Freaks. A little bitterness just might take root there, and fester over the course of an immortal existence.
Violation of the Seven Laws of Magic in The Dresden Files causes this, especially things that harm other people like mind control, Outsider summoning and murder-by-magic. It not only corrupts your soul but eventually drives you insane with power-hunger (assuming that the Wardens don't kill you first - at first you think that's the Wardens being draconian and cruel, but it turns out the laws and their one punishment exist because once you do certain things, you will get worse and worse and it won't matter what your intention was when you started. They don't like the fact that the best way to keep the body count down is to get out the ax when the violator is still a poor kid who broke a law once without knowing about the laws or council or wardens, but it is. The council basically answers "yes" to "knowing what was coming, would you kill Hitler while he was stilll a baby?").
More personally, Harry is constantly tempted to use Fallen power by Lasciel, who wants him to pick up her coin and become a Denarian. At one point he even quotes Gandalf at her to emphasize why he refuses.
The Denarians are perfect living examples of falling to The Dark Side. They're evil through and through because of their greed for power, and they live symbiotically with demons from hell (in the literal Christian sense).
The only member of the White Council that is allowed to violate the Seven Laws is the Blackstaff. This is because the Blackstaff (magical staff) wielded by the Blackstaff (the position) prevents The Dark Side from corrupting its wielder.
In The Tennis Shoes Adventure Series, there's a demonic sword, created by an evil king from The Book of Mormon. It would slowly work its way into someone's mind, convincing them they could use it to become powerful. Through the use of Time Travel, Todd Finlay got a hold of it and turned to The Dark Side. Later on, Joshua Hawkins, son of The Hero gets seduced by it as well.
The so-called True Power of The Wheel of Time is highly addictive. Only the very upper echelon of the Dark One's flunkies are allowed to use it, and only one of them does so on a more than completely desperate basis, as they're all afraid of the side effect: going utterly around the bend (the one who does use it is already nuts and a major nihilist to boot- he doesn't care about the consequences]].
Saidin, the source of power for male magic users, is also considered to be this in-universe, but that's because the Dark One managed to layer corruption over it, causing anyone who uses it for long to go completely Ax Crazy and then die a horrible death that's never seen, but described as being essentially rotting while you're still alive. Late in the series, Rand maanges to fix it.
Although they were not good to start with, Severus Snape, Regulus Black, and Draco Malfoy are all examples of characters who liked the idea of the Dark Side, joined the Death Eaters at the first chance, and had gotten too far in before they realized what it was really like.
In the Drizzt novels by R.A. Salvatore, the Crystal Shard is frequently presented as being made of the Dark Side. So too the sentient sword Khazid'Hea. The Crystal Shard is an interesting and somewhat tragic case, since the true source of its corrupting influence isn't from the evil beings that created it (whom it consumed), but from the lingering sorrow and torment of a ruler who tried and failed to protect his kingdom from invaders with the Shard's power.
In Randall Garrett's Lord Darcy universe, practicing Black Magic for any amount of time will result in this.
In Mercedes Lackey's Valdemar Universe there are hints that the ancient Big Bad Ma'ar may have started out this way.
In Captain Power and the Soldiers of the Future, Lyman Taggart wanted to use the supercomputer Overmind to end the ongoing war being proliferated by the Biomechs, but was seduced by the seeming perfection of machines, inevitably becoming the infamous Lord Dread and the very thing he was fighting against. His occasional doubts are dissuaded by Overmind, who brainwashes him with the propaganda of their cause.
The Wraith Enzyme in Stargate Atlantis, though Ford has yet to venture into full-on Darth Vader territory.
And probably never will, because his character has seemingly been forgotten by characters and writers alike.
While they Never Found the Body, the implication that Ford died in his last appearance was fairly obvious. They left just enough of an opening for him to be brought back if the writers decided to do so, but they never did.
Subverted in that you have to go through the Dark Side, survive (not explode) and come out the other end to be on the real Light Side.
Also, in season 3, Angel warns Faith "You don't know the price of evil". Her response: "I hope evil takes MasterCard".
Angel's conversions to Angelus don't count as they are instant and involuntary. But he goes dark in season 2, in a tantrum over not being able to save Darla. For example, he lets a room full of lawyers get eaten by vampires. That's dark, isn't it? Isn't it?
Spoofed in Season 6 when Spike encourages Buffy to walk with him on the Dark Side — which consists of Spike playing poker for kittens while Buffy gets drunk and makes snarky comments. Things become more serious later on in the season when Spike wrongly assumes (or convinces himself) that Buffy's depression and desire for rough sex means she wants to abandon her life and join him on the Dark Side. His failure to understand the complexity of her emotions has serious consequences for both of them.
The darker aspect of Delvian mental powers in Farscape, especially in the episodes "That Old Black Magic" and "Rhapsody In Blue". All in all, Delvian priests have a worrying tendency to turn to the Dark Side, for as they train for purity and gain more control over their abilities, they become more vulnerable to darker impulses; if they succumb to these, they will very quickly go insane.
The Shadows in Babylon 5 start their work by tempting and corrupting powerful people.
The Secret Society in House of Anubis soon bring in the newest teacher into their ranks (leading to his face heel turn.
Season 3 has, what the heroes call, Team Evil. They eventually also start corrupting people into their team, by locking them into a sarcophagus and stealing their soul for the god Ammit to devour. They are even worse when you discover they took the soul of one of the Sibuna Members themselves. While this could be considered mind control, the members who had their soul lost openly admit that power feels good and are all glad that they lost their conscience, while the rest of them feel that way anyways.
New World of Darkness is filled with powers that slowly corrupt and tear into those who call upon them. In many cases, simply being supernatural leads one into temptation — the average Humanity of an elder vampire is around four, which is about the point at which even murder doesn't look like that much of a big deal anymore.
The Gangrel clan from Vampire The Requiem get their own special variant. Any vampire can "ride the wave" and give into their Beast, but Gangrel can enter "the Red Surrender," a state where the Beast takes over and they become much more powerful. The downside? It's addictive.
You can break the addiction though through sheer willpower, and end up becoming a  you become more connected to your humanity by telling the Beast where to stick it, and get bonuses to mental stats (Called the Red Denial) the downside? It feels about has pleasant as hearing nails on a chalkboard, and chewing tinfoil at the same time. Denying the beast feels AWFUL.
Werewolf The Apocalypse has the Wyrm, the spiritual embodiment of entropy, death, and necessary destruction. Or rather, that's what it used to be, before it went batshit insane and became an Omnicidal Maniac. Nowadays it's a force of subtle corruption and despair, an army of not-at-all-subtle mutants and sadistic monsters, and a spiritual energy fed and strengthened by every atrocity and act of abuse in the world. Its spirits animate countless tainted objects and places all over the Earth. Every emotion of hate, and every act of destruction and abuse a person commits, slightly erodes their soul and makes them more open to the Urge Wyrms, who push them further towards their master. Werewolves who ally with the Wyrm (willingly or not) become sadistic, insane, mutated, fucked-up Black Spiral Dancers.
Mage The Ascension has the various evil forces served by the Nephandi, some of which appear to come from the Wyrm and others of which seem to be Eldritch Abominations from the far edges of the Solar System. Serving them is not healthy for your sanity.
The Elohim in Demon: The Fallen have the Torment: the mark of their estrangement from God and/or Grace. Due to the nature of the setting, it's much easier to gain Torment than to absolve oneself from it. And the more Torment you have, the more likely you are (it's in the gameplay mechanics!) to screw up some benevolent deed and turn it into a crime, earning more Torment in turn...
Somewhat predictably for the setting, Warhammer 40000 takes this trope and hurls it screaming over the edge in the form of Chaos. The result? There's no Light Side—only a sort of Gray Side, and the actual Dark Side is sentient. A sentient, extremely intelligent, masterfully manipulative, and very powerful Dark Side that occasionally takes matters into its own hands when mortal pawns aren't getting the job done.
Happens quite a bit to Radical Inquisitors in the background, using demonic powers or weapons to fight Chaos is a good way to get corrupted to Chaos, especially because many Radicals have to employ evil methods to acquire such power.
An excellent example of the power of Chaos, is the Horus Heresy novel Fulgrim. Fulgrim starts out as a Primarch, the settings' equivalent of a human archangel, and a great lover of art of all kinds. In the end, the least of his depravities is creating art with the blood and screams of his victims. Throughout this, Fulgrim never realised what was happening, and every step on the way makes absolute, reasonable, bone-chilling sense.
Tends to happen when you start using spells with the "Evil" descriptor in Dungeons & Dragons. Note: the descriptor is there for a reason.
The "Evil" descriptor is a bit misleading at times; Spells such as "Phantasmal Killer", which is Mind Rape in everything but name, does not have the descriptor, while spells such as "Deathwatch" which gives you a little timer to tell when the person you cast it on dies, does.
In Dungeons & Dragons evil isn't just a word but a literal component of existence, which means that evil spells use physical capital-E Evil as an ingredient. Casting Phantasmal Killer against a group of children would still be evil, but since it doesn't actually invoke Evil as a spell component, the spell itself is as morally neutral as a Fireball or Lightning Bolt.
Like many depictions of Angels, should In Nomine Angels go against their nature too much, they Fall and that Fall changes their very essence into Demons. It is up to the players how easy this is.
It is in the very nature of Ravenloft, the Demiplane of Dread, to addict and reward whoever acts evil.
In fact, Ravenloft doesn't reward evil, it actually punishes it horrifically, by dangling the evil person's greatest desire just out of reach. For all eternity.
Ebon Dragon Infernal Charms, in Exalted. There are a lot of other morally dubious Charms, spells, and supernatural abilities, but only the Ebon Dragon's Charm trees will steadily impair your ability to act in an honest, moral fashion as you progress along them. (Granted, Infernals are one of the "villainous" factions to begin with, but they're entirely possible to play heroically, and Eclipse Solars and Moonshadow Abyssals can learn their Charms...)
Blue Rose: Sorcery — meaning most forms of ritual magic, as well as ordinary Inherent Gift magic if used in certain unethical ways — works almost exactly like the classic Star Wars Dark Side.
Picking up the theme from its literary counterpart above, in the Dresden Files RPG breaking any of the Laws of Magic invites attention from the Wardens and pushes player characters closer to NPC status...but each "Lawbreaker" power (one per Law and automatically acquired upon breaking it) then also provides a bonus towards actions breaking the same Law of Magic again.
The powerfully corrupting effects of the Dark Side of the Force feature in Star Wars video games.
For instance, in the game Knights of the Old Republic, you are eventually confronted by a fallen Jedi, Bastila, who attempts to lure you to the Dark Side and applauds atrocious actions... despite the fact that they were a dedicated Jedi Knight only a short time ago — no more than four to six hours of gameplay time.
Although it's mentioned that Bastila was brutally tortured by Malak for over a month before he managed to break her and convince her to embrace the Dark-Side, so it's somewhat justifiable.
In the alternate ending of The Force Unleashed, you can choose to kill Darth Vader and take his place as the Emperor's right-hand man. However, you then decide to kill the Emperor to elevate yourself further, but he sees it coming and mangles you to the point that you had to be put into a suit like Vader was, and are forced to serve as his apprentice indefinitely (Palps is also noticeably much more blunt about his opinion of you than he was about Vader).
In Jedi Academy, the reason given to why your character is tempted by the Dark Side is his/her mounting anger towards a sort-of-friend who has been annoying and trying to beat him/her all along, culminating with trying to kill him/her during his own momentary fall to the Dark Side. If you take the Dark Side option and have Jaden kill him, then instead of maybe going My God, What Have I Done? or something else sensible after his/her anger fading, (s)he will go on to decide (s)he now wants power above all else and go after the villain not to stop her but to steal her Artifact of Doom. Apparently for a Jedi to become angry enough to kill a helpless prisoner automatically also makes them obsessed with power and others' weakness.
The character Arthas follows this trope to the letter. Originally a paladin, he obsesses more and more over defeating the Undead Scourge that has been appearing throughout the land. When he finally traces it to its source, he decides to go after a Infinity+1 Sword, Frostmourne, to defeat the undead menace. When he gets the sword, it murders his friend, steals his soul and whispers in his head, driving him mad and leading him to the Dark Side, eventually causing him to slay not only his family, his mentors, but overrun the land of his birth completely, which was what he was trying to prevent.
Oh, yes, and to become the master of the Scourge himself.
Then there is the Nightelf Illidan Stormrage, who had a similar path except for more personal reasons coupled with the effects this had on his body causing his peers to mistrust him. Ten thousand years in prison didn't help things, as he immediately (though manipulated by Arthas) went to claim a demon artifact that turned him into a half demon. He eventually allied with the demons in an attempt to stop the Lich King, and went completely insane when he was heavily injured while trying to do so.
Might be averted due to Thrall and the Dragon Aspects sending adventurers to the past in World of Warcraft to retrieve what could seemly be the artifact itself, the Dragon Soul, to use to kill Deathwing in the 5-man dungeon "Well of Eternity". Although one of the Aspects said that the Dragon Soul was sent back to its rightful place in time after Deathwing's defeat, there were hints given by Blizzard to indicate that removing the artifact from that point at all could have severe consequences.
Warlocks and Deathknights in World of Warcraft, using evil powers to fight evil, are basically under constant threat of this happening, and there are many boss characters that have. Players are immune to this.
Of course, there are the Old Gods that don't exactly fight battles on their own, but rather drive others mad, such as Deathwing.
Finally, managing effects similar to this trope has been a key element to a few fights, most notably the Bloodqueen in Icecrown Citadel turning players into vampires. Affected players are much more powerful, but eventually have to bite someone else or they'll be mind-controlled.
In Spore, you can choose to ally with the Grox, which actually gives you the "Dancing with the Devil" achievement, with the description "Have we gone to the dark side now?"
The Satsui No Hadou or "Killing Intent" from Street Fighter, which both Ryu and Ken have to deal with and which Akuma has embraced.
In the last route of Fate/stay night, Angra Mainyu's corruption of the Grail causes Sakura to undergo this kind of transformation after she suffers an Attempted Rape by her adopted brother. Any of the Servants she corrupts instead of killing outright, including Saber and Berserker, undergo a shift to The Dark Side along with her.
In Kingdom Hearts, this is represented by the "power of Darkness", which eventually corrupts all of the main villains and The Rival Riku. Later games establish that Dark Is Not Evil (sort of), allowing Riku to retain his good guy status and his cool Darkness powers.
It was also stated that Riku really is a special case, Darkness always corrupts and recovering from it to the extent Riku did has only ever been done by him and him alone, And even getting there still took him inheriting his powers from someone who was falling to the darkness when he was given the spark and losing his body to it, and the manifestation of his inner darkness still haunts him five games and three years later.
'"We are Lunite. Hatred is our lifeblood... Rancor shall pave the path to the crimson heavens!"'
Worms has a rather odd take on the Darkside being defensive by burrowing into the terrain and gaurding yourself with girders, it's still fighting dirty as you can burrow down and unleash an Armageddon if you have one.
Gwynn from Sluggy Freelance has this with the Book of E-Ville and (formerly) with the demon K'Z'K.
Subverted with Vaarsuvius in The Order of the Stick and his/her soul splice with the spirits of three psychotic mages in order to stop the black dragon threatening V's mate and children, but also out of pride. Vaarsuvius was told the soul splice would have an effect on alignment, but after the Moral Event Horizon is crossed, this is revealed to be a lie set up to give V an excuse to do something that the elf would have otherwise avoided.
Charmander, after using Rare Candies, becomes a bully Charmeleon and almost aggressively attacks Jigglypuff in 151 Hidden Depths.
Parodied and subverted in Homestuck. After her mother dies and she learns something about theHorroraterrors Rose goes "grim dark". Although this changes her appearance and prevents her from speaking normally, her personality is seemingly unchanged...though she understandably wants to kill the Reality Warper who killed her mother.
Firebenders have all been taught to fuel their bending with anger, enhancing their warlike tendencies and giving many of them anger management problems. Late in the series Zuko learns to use the sun instead, and Iroh may have been doing the same all series. Even Aang's first Firebending teacher viewed it as a wholly destructive power.
Even Waterbending can have darker uses, as in being able to Bloodbend any living creature, or rip out the water within them.
Parodied in Spongebob Squarepants when Barnacleboy tells Mermaidman he's crossing over to the Dark Side. It's then shown that half of the Krusty Krab restaurant is in a veil of utter blackness, to the point where we can't see anything within.
Mr. Krabs: "Why should I pay to light the whole restaurant?"
Raven in the first season Teen Titans episode "Nevermore". Her inner darkness took control of her, that's why she pulled Dr. Light under her cloak with all the black tentacles and did something horrible underneath. All with glowingred eyes and the nastiest smile she's ever had in the show. It's a deep internal struggle though, and she's mortified when she snaps out of it. Though the glow comes back and she snarls with sharp teeth when asked what she'd done to him.