The original novels that the Slayers anime is based on, while still fairly comical, are far more violent and grim. A particular spell, "Ragnut Rushavna", is gruesome to the point that no other media has had it: The spell involves the victim to be turned into a giant hunk of flesh that is eaten repeatedly by snakes that spontaneously generate from it over and over again until the caster of the spell is detained. The characters themselves, other than Lina ironically, are less humorous and blase than they are in the games or the anime.
On the flip side, Yu-Gi-Oh! 5Ds went the way of AKIRA in order to achieve Darker And Edgier. In fact, popular consensus among the show's fans is that the sheer spike in Darker And Edgier material in it was a deliberate act to keep 4Kids Entertainment from Macekreing it... not like it hasn't stopped them from trying.
Speaking of Yu-Gi-Oh!, Yu-Gi-Oh! GX went from the brainwashing cult of season 2 to the way darker tone of season 3. It started out with new main characters as usual and a seemingly tame (at least compared to season 2) villain until he gets defeated relatively quickly and then the series gets much darker including the characters dying when they lose a duel, at least that's what they are told, the main character getting tricked into releasing his Superpowered Evil Side and killing some of his remaining friends, and the The Man Behind the Man/Stalker with a crush/Hidden Agenda Villain possessing the main character's best friend. Although 4Kids Entertainment ruined season 3's dark tone in the dub, watching the Japanese version makes the dark tone obvious.
Little known fact: AKIRA was conceived as a Darker and Edgier retelling of Gigantor.
The actual Tetsujin remakes also have a touch of this. While the 1980s version wasn't particularly dark, the look of the series was redesigned to make it less cartoony and whimsical and more like the modern, high-tech Super Robot shows of the day. This ultimately resulted in a rather bland show and it's very telling that when a sequel series, Tetsujin 28-FX came out in the 90s, despite the new robots being even more complex, overbuilt and gimmicky, with transformation and combination gimmicks and such, the original Tetsujin was drawn in the classic style, googly eyes and all. The second remake in the 00s, from the people who brought you Giant Robo, took it in a whole new direction, keeping the cartoonish 1950s visual style and wrapping it around a grim story about war and political intrigue... and a boy detective with a giant cartoony robot.
Meg not only lost fights, but she had to face serious consequences (deaths, injuries, humiliations, etc.) that could 'not be undone with magic.
Every adaptation of Read or Die seems to do this. The manga is fairly light-hearted (and the Read or Dream manga entirely so). The OVA has a bit of camp to it, but gets fairly dark, with a Bittersweet Ending. The TV series, R.O.D the TV, manages to get more depressing nearly every episode, but eventually rewards its long-suffering cast with a happy ending. Strangely, they keep improving.
Tekkaman Blade is a Darker and Edgier version of an earlier series called Tekkaman the Space Knight.
Pokémon Special, a manga spinoff of Pokémon's video game series, has fights to the death for everyone, humans included.
Pokémon: Diamond and Pearl Adventure is this in a way. Not in terms of violence, though it has been described as "Dragon Ball meets Pokémon" and is hefty on the fighting, but more-so in the plot. Especially when Mitsumi and her past is involved.
As far as the Pokémon anime goes, Diamond/Pearl and Best Wishes are both noticeably darker and more serious at times than much of what came before it, some even surpassing the games, though the original Kanto season had its fair share of dark moments as well. Still, it is generally lighthearted and makes it clear that it is aimed at a young demographic.
Pokémon Origins has 4 episodes, but shows it's definitely darker. The fighting is more brutal involving worse attacks, blood-curdling screaming from getting bitten and almost dying, or being beaten to death by a stun baton, and a Mewtwo that is portrayed as a powerful psychotic monster.
The movies in general are this, most of them having at least one of these features:
A subject that's more serious or complex than usually found in the TV series.
Urasawa's Pluto does this to Tezuka's Astro Boy, showing the grittier side of life in the twenty-first century. This being Urasawa, it works tremendously well as the writing and themes are thoughtful and touching as well as dark and edgy.
Arguably a subversion. Urasawa doesn't change the basic plot very much at all, resulting in most of the same major robots dying as did in the original. ie all of them. What made the story darker was the fact that each robot was actually developed, rather than simply being a collection of powers to be destroyed by Pluto.
GaoGaiGar FINAL was much darker than the television show for two main reasons: first, it was an OAV so they could get away with things that wouldn't fly on broadcast television, like explicitly sexual Fanservice. And secondly, it was aimed at a seinen audience rather than at children.
Martian Successor Nadesico has Prince of Darkness as its Darker and Edgier Title, the plot becomes a moodier and much darker plot of the original series as many a dream is destroyed and it was not well received by fans at all
Higurashi: When They Cry utilizes this about every 5 or 6 episodes. Each arc begins with a few gruesome shots of one of the character's deaths, and then proceeds to be lighter and happier for a while, while steadily approaching the doom shown at the beginning of the arc, getting darker as it goes on.
Umineko: When They Cry not only follows a similar formula, but also has this trope by actually BEING darker and edgier than Higurashi. At least Higurashi was limited to depicting things that are relatively down to earth. Umineko gives us witches who do whatever they want, often to brutally graphic extents. One even delights in killing her victims in brutally creative ways, only to revive them and do it all over again. And when they're done with the killing they'll start with Mind Rape that makes Higurashi look like Casey And Friends.
In One Piece, the 6th movie, Baron Omatsuri and the Secret Island, the Straw Hats start to separate from each other, due to conflicts the antagonist is creating. The plot also contains some of the biggest horrors, as Luffy's comrades are eaten by a Man-Eating Plant called "Lily", the Big Bad's backstory is also different, showing that his crew died, and he found Lily, allowing him to create Replacement Goldfish, so long as he feeds Lily, the movie's plot is also slightly more complex.
In the series proper, we had the Water 7 arc, easily the darkest arc in the series the Merry Go is deemed unfixable and can't sail any further, Usopp leaves the crew when the news is broken to him leading to a fight with Luffy, Robin disappears and later aids in framing the Straw Hats (though not without good reason) and the crew is nearly killed upon meeting the CP9. Oh and of course Robin's flashback the followig arc. Likewise the last bits of the first half have been leaning toward this the such as Ace finally finding Blackbeard and losing the introduction of World Nobles, the crew getting beaten by a Admiral and getting separated from each other. Luffy finding out about Ace's execution and breaking into Impel Down to rescue him, not only failing but nearly dying in the process. Then of course the whole war that followed to rescue Ace that ultimately ended with Ace dying, Whitebeard, one of the most powerful Big Good pirates on the planet being killed by Blackbeard and having his earthquake power stolen. Luffy actually doubting his dreams and a flashback that showed the (supposed) death of a close friend of theirs. Granted Luffy recovers and the crew reunite but even then dark times are on the horizon as Akainu, the Admiral who killed Ace, is now head of the marines. So...yeah.
Naruto Shippuden/Part II has been increasingly darker than the original series. The main character, Naruto, loses the closest thing he has to a father, Jiraiya, his village is nuked, his mother figure Tsunade is knocked into a coma, he is put into a Breaking Speech by Pain which he couldn't retort to... even though he managed to get Pain to bring everyone back to life that he killed that day, the Village is still a crater, Tsunade is still in a coma, and Danzo nearly became Hokage. THEN we get to Sasuke, his former rival and best friend who had joined Akatsuki and attacked Killer Bee, and then attacked the Kage Summit. Learning how far his best friend had fallen caused him to have a Heroic BSOD and fall into an Angst Coma. Shippuden/II has more death, destruction, horrors, and Orochimaru-and later Obito creepiness to show it's much darker and edgier than Part I.
Digimon Savers is also a good example, thanks to emulating the above show.
In-universe example in Ouran High School Host Club: Renge's first appearance has her making a video where the normally-cheery members of the club become darker personae, e.g. the Keet becoming a callous bully. It sells well enough to take the club out of debt.
Karakuridouji Ultimo seems to follow a zig-zag pattern with this trope and Lighter and Softer. The series started fairly light hearted, and comical, up until the chapter where Ultimo, the embodiment of good, beats up an evil doji into a rather nighmarish pulp while smiling. Then they threw in the main character's best friend turning into a Yandere, the world blowing up, and the entire good cast being killed quite brutally. Once everyone got better in Part 2 of the series, it went back to its original state, but then implied to have killed off most of the original Good Doji Masters. Then Part 3 got even worse, by having the earlier mentioned Yandere nearly rape the main character in a rather dark manner.
Once in The Seventies, there was a short shonen manga by Kazumasa Hirai and Hisashi Sakaguchi. Its name was Wolf Guy, and it was about the adventures of young werewolf Akira Inugami and his Hot Teacher Akiko Aoshika. Decades later, Yoshiaki Tabata and Yugo Yuuki (the authors of Akumetsu) took the basic concept of this manga and re-made it into a seinen story named Wolf Guy - Wolfen Crest - which is full of Gorn, Fan Disservice, etc. Inugami gets almost killed several times, Aoshika-sensei is almost completely broken in all senses, and Big Bad Haguro Dou goes from a mere Yakuza heir to one of the most despicable villains in manga.
AD Police, the spin-off of Bubblegum Crisis, is darker, more violent and adds some psychological spins to the original series concept. The TV spin-off they produced to this spin-off a decade later surprisingly was closer in tone to the original Bubblegum Crisis.
Osamu Tezuka's Black Jack is a rather dark series to begin with, safe for some comedic touches usually provided by Pinoko. Osamu Dezaki's video adaptation in the 90's however takes the darkness to extremes and (for the most part) deletes all hints of humour in the story, opting for high-tension, highly stylized drama instead.
Arguably happened to the anime installments of Magical Girl Lyrical Nanoha. The series starts out as an archetypal, formulaic Magical Girl series, but then the first season turns it into more of a serious action series with magical girls. The second season, meanwhile, goes from having no real villains to uncovering a Sealedcyborg (who ends up being an Anti-Villain anyway). The third season then becomes a straight military series with a little bit of magical girls.
The fandom tends to do this. It seems everyone wants it to be Darker And Edgier. Then, there's the popular BetrayerS doujinshi, which cranks the darkness and edgy-ness up to 11. Hayate and Nanoha being Lawful Evil. You can get it here.
It should be noted that the fan response to the actually-darker FORCE, mentioned below, has been mixed at best.
Record of the Magical War Nanoha Force is said to be Darker and Edgier by the writer. And he wasn't kidding. In the 3rd Chapter, we were introduced to Card-Carrying Villain Veyron who introduces himself by killing everyone in a chapel that Touma had visited earlier. We won't see Nanoha fall to the dark side but definitely we will see more people being offed by Belkan weaponry then usual and maybe a few kills with Midchildan weapons. Then, a few chapters later, there's a brutal fight between Signum and the Dark Action Girl Cypha, which involves the latter being partly dismembered and the former, landing in a pool of her own blood with a sword in her gut.
And how Touma's device is relies on The Corruption to work, and Lily is a source of death and ruin as she accidentally causes people to degrade to Body Horror like meat blobs
And now Rebuild of Evangelion joins the club with its third installment, moving the formerly Lighter and Softer remake right back into depression territory with Shinji's little stunt at the end of the second film wiping out most of the human race and reducing him into a near-universally hated pariah for almost every other cast member, not to mention Kaworu sacrificing himselfyet again to prevent Fourth Impact.
Gundam SEED, to frankly put, is extremely dark for an Alternate Universe Gundam series. You got yourself brutal atrocities committed by both sides which becomes a Guilt-Free Extermination War with very little in the way of clear comic relief. There is no mercy, no honor, no salvation for anyone. Civilians and soldiers are equally expendable, and not even one's own allies can be trusted. Victory for either side means virtual destruction of an entire subtype of the human race and by the end, all of it.
Oh, and the explicit horror isn't where it ends. You know all that genetic manipulation that forms the backbone of the series? Yea, turns out that the entire thing was taken to the logical extreme by essentially using live humans and clones as disposable test subjects. Not only did this fail to produce an implementable solution to the coordinator issue, it spawned a living representation of all of mankind's sins against itself, the last clone Rau Le Creuset, who is determined to enact a sick, yet horrifyingly justified restitution upon the human race.
Gundam 00 took this closer to home by placing the series in the Anno Domoni timeline where most of the world's current problems aren't just still around, they're exacerbated, and it's going to take a lot of blood to get any work done on them at all. This also makes it one of the darkest alternate universe Gundam, up next to SEED.
Until the movie suddenly snapped everything into hope and rainbows, giving us one of the few unequivocally happy endings in Gundam. It even has world peace.
Gundam AGE unveiled its true colors from episode 14 onwards. Similar to Madoka, don't let the series' kiddy art designs fool you. While initially masquerading as a children's Gundam series for the first 13 episodes, the Love Interest gets brutally killed in episode 14 and The Hero consequentially takes the dark descent into becoming a genocidal Knight TemplarWho Fights Monsters. Oh, and the UE are Human All Along. And those hoping for a bloodless final episode was hopelessly crushed at it ends up in a Tomino Kill'em all for BOTH sides. Add the almost-complete absence of comic relief and the series being set in a Crapsack World that would feel right at home in the Universal Century or Cosmic Era, and you got a series that somehow manages to get up there with SEED and first part of 00 as the darkest AU series yet.
Where the Gundam series is concerned, it's sometimes darker and edgier in their side stories, regardless of the calendar. Comic relief aside, the overall progression of arcs, foreknowledge, the ending and themes show that the likes of Mobile Suit Gundam 0080: War in the Pocket and C.E. 73 Stargazer to be less sunshiny than their respective calendars' main series.
To a point Gundam franchise in general was darker and edgier in comparison to the other mecha anime around, with the themes, the (generally) Grey and Gray Morality, the various War tropes present and the franchise was started by a man known as "Kill 'em All".
The FLCL manga is much darker and more bizarre than the anime its rather loosely based on. An example of this would be Naota killing his dad with a baseball bat.
Ironically, the manga has a lot of dark elements that weren't brought over. The Ishvalan War in the anime was a short Curb-Stomp Battle, while in the manga it's full-on War Is Hell and a number of chapters are devoted to showing how horrible it was. Also the fact there's two (although the second didn't take) counts of national genocide and unlike the Ishval war, The survivers could be counted on, at the most, both hands. The anime may be more cynical overall, but the manga has far more (near-)deaths, most likely because the war was not shown yet in the manga when the anime was being made.
Front Mission is known for being a war drama, but Dog Life and Dog Style takes it Up to Eleven as it incorporates the grittiness of Red Eyes in a completely uncensored and brutal manner (Which relates the journalist who brings out the uncensored truth). In the first two issues of the manga it starts off showing gruesome death, rape/sex and ultra violence.
The S Season, which is overall darker than its predecessor. While overall pretty light-hearted, featuring some progressively sillier monsters (as well as Eugeal's car appearing in progressively sillier locations.), once Mimete's removed from the picture, the season takes a dark turn. You know a season's about to get real when the final group of villains don't even bother to summon the silly monsters of the week their predecessors summoned.
The villain Queen Nehelenia appeared in two seasons, and the contrast between which is huge. Season 4 was mostly rather light-hearted: there was a pegasus, the villains were campy circus people who were commanded around by a weird old lady, and it focused mostly on Chibiusa; only at the very end there was a real difference. The first part of season 5, however, was entirely different, even breaking with the usual format. There were no monsters of the week, only Nehelenia's nearly indestructible mirror minions. In the course of the arc, Mamoru was brain-washed once again, resulting in Chibiusa almost fading, and all the Inner senshi were taken out one by one, sometimes in very painful manners (like Makoto being electrified almost to death by Nehelenia herself). This arc also brought back the outer senshi, who have a much more cynical approach to these matters... and they also get taken out. Usagi barely manages to win, but before that she has to go through a full-blown Break the Cutie process that almost throws her to the Despair Event Horizon. (And how does she win? Via offering herself to be tortured and killed by Nehelenia, who also shows heartbreaking Hidden Depths as we learn about the process that made her... well, the way she was. Only then things start going back to Lighter and Softer.)
Even more so, the last part of Stars. Where after a long search, the Sailor Starlights are reunited with their Princess... only for her to be killed. Then the Inner Senshi get killed. And we find out Mamoru has beenDead All Along for a while already. AND Haruka an Michiru kill Setsuna and Hotaru as a part of a Fake Defector plan, but fail and die at Galaxia's hands. And we're not done...
Fate/Zero is a Darker and Edgier prequel to Fate/stay night — intentionally so, since the novel's intent is to show just how dark, depressing, and violent the Holy Grail War can be. This was to be expected, because the parts of the backstory already revealed Fate Stay Night make all but a tiny handful of the Fate Zero characters Doomed by Canon.
The Movie installments of Doraemon can easily feels like this, even if Nobita and friends remain good people. One movie deal with the horrid effect Black Magic (in a magical world), which comes from devils (who are aliens). Another deal with the revenge of the Reptilian people evolved from dinosaurs, who want to alter time so humans never exist. Yet another deal with the horrors of war as terrifying aliens (who are humans in hazmat suit) invade a utopian planet inhabited by cute humanoid animals. The last one has a particular scene that can't be anything else but traumatic to its supposed audience.
THE iDOLM@STER - while the first half of the series was very lighthearted, the second half progressively began to take on darker tones, culminating in 765 Pro's rivalry with 961, Chihaya's dead little brother plot, and the Producer getting into a life threatening accident.
Lupin III, which we include because a fan is more likely to be familiar with the anime equivalents, rather than any of the comics that came first. It establishes itself as a series with sex, violent death, and occasional Gorn.
The annual specials can spend some of the time Darker, and some of the time Lighter.
Lupin III: Dead or Alive is the darkest of the Lupin Theatrical Releases, and has the notable honor of Monkey Punch working on it.
If you thought that Yuu Watase's Fushigi Yuugi Genbu Kaiden was merely Fushigi Yuugi from two hundred years ago... think again. Becaue not only the heroine Takiko comes from a very different time in the real world (more exactly, Imperial Japan during The Roaring Twenties, in contrast to Miaka and Yui who come from the Japan of The Nineties), but the Shi Jin Ten Chi Sho that she roams through is far bloodier and more complicated than the one we viewed during Miaka and Yui's journeys.
Similarly, Sakura Gari is both Hotter and Sexier and this. Not only it has Ho Yay by the wazoo and explicit sex scenes, but also massive doses of dark themes like incest, rape, physical abuse, psychological abuse, etc.)
Bleach was already one of the darkest titles in Shounen Jump but the Thousand Year Blood War has turned this Up to Eleven. This was, notably, right around the time the anime got cancelled and Kubo Tite no longer had any worries about network censors. Suddenly, good guys can die (usually with gaping holes where part of their body used to be), adhering to Honor Before Reason shonen battle conventions will only get you killed, and the Big Bad is responsible for a genocide OF HIS OWN SUBJECTS.
Mawaru-Penguindrum starts off like a comedy where three siblings have to deal with invisible penguins and get a girl's diary, but Ringo eventually tries to rape Tabuki, Kanba becomes a terrorist to save Himari, Yuri tries to rape Ringo, Shouma can't cope with how his and Kanba's parents seemingly caused the death of Ringo's sister, etc.. At least she changed her mind.
The World God Only Knows, A light-hearted manga-series about a Otaku playing video-games and using that video-game knowledge to help women in real-life get over their problems which in the process leads to the creation of an Unwanted Harem. Read the early chapters that highlights the before description, and then skip directly to chapter 214. You'll be amazed at how dark the manga becomes. To elaborate, what starts off as a comedy/romance manga series, slowly begins to showcase Shounen-style action during the later portion of the Goddess arc, and then delves further to even showcasing blood-and-gore once it reaches the Past arc.
Zigzagged in the Pretty Cure franchise. Some installments are dark, but most are light. The dark ones usually have one or more of the eponymous magical girl having to deal with personal failure that cost them dearly. The reigning champion of Grimdarkness is currently Heart Catch Pretty Cure, with Cure Moonlight having to deal with her Fairy Companion dying in front of her, her father being The Dragon, her father killed by the actual Big Bad in front of her, her Evil Counterpart is actually kinda-sorta her little sister, said little sister dies in front of her by her own hands, and then figuring out that even saving the world won't bring them back. Hurting Hero, she is.
DokiDoki! Precure seems intent on One Upping the above, by not only having an expy of Yuri, but by also showing, onscreen, an entire kingdom being devastated by the Big Bad (Previous seasons merely gave a small glimpse of how the big bad operated and the devastation in his wake), as well as being the only series in the franchise to date to subvert one of the then standard formula practices in the series, in that a Dark Magical Girl, when redeemed, automatically stays on the side of good as a Precure in Regina, only for her to almost immediately be forced back onto the side of evil by the big bad, who's One of the most evil Big Bad's since Gooyan... and he's not even the Big Bad that appears for most of the series and is in fact controlling him. Damn.
The Noozles took a grimdark turn about halfway though, with the introduction of Koalawallaland, its human-hating Nazi Koala inhabitants, and the threat of a Reality-Breaking Paradox due to the separation of the two universes being accelerated by seismic testing.
If you only know Yoshitomi Akihito for his kawaii Yuri short stories, then School Mermaid, a horror manga, will take you by surprise.