In comics, this move is most famous for Batman. After the end of the Batman TV series, it became apparent the campy tone had burnt out, and DC realized a change was needed quickly. With Denny O'Neil's writing and predominantly Neal Adams's gothic and realistic art, Batman was made a darkly fearsome night stalker much like he was in the original stories before he was softened for kids. Later, in the mid-80s, Frank Miller's Batman: The Dark Knight Returns overclocked this to dangerous levels. The pattern repeated itself with the movies. After the increasingly silly Batman Forever and Batman & Robin movies failed, DC started again from the beginning with the more serious Batman Begins.
The shift also carried over to Batman's Rogues Gallery, most notably The Joker, who had been written as a comical "Clown Prince of Crime", but now returned to his psychotic murderous roots and building up one of the largest body counts in the DC Universe (only being outdone by alien societies and villains with near-god level power).
In the '90s the Batgirl mantle was passed from Barbara Gordon to Cassandra Cain, a character who came complete with a much darker origin (she's a mute trained from birth to be an assassin) and a costume that wouldn't look out of place at a BDSM club. Fortunately, she was written well enough in her own series to not come off as ridiculous.
Bat-Azrael was a darker, edgier, more brutish version of Batman, created to show what makes the true Batman not a vigilante. However, DC was ready to keep Azrael as Batman, if it sold well enough.
Jason Todd as Batman is similar to Azrael: a thuggish, heavily armored Batman who guns criminals down with his pair of pistols. Fans have taken to calling him "Gunbats".
From Death Of The Family, a number of people might laugh and say that Joker can't possibly achieve this trope at this point. They would be wrong, because his treatment of Harley Quinn is even worse than it was before!
Speaking of Alan Moore, he actually did a Darker and Edgier reboot of UK superhero Marvelman as well. What had originally started out as a British Captain Marvel rip-off, turned into a gritty, Total Recall-ish, what-is-real head trip, that even turned his Freddy Freeman-esque sidekick Kid Marvelman into a violent psychopath.
The Transformers Generation 2 comic books, loosed from even the moderate Contractual Immortality restrictions they had been operating under before, promptly started massacring the cast. Issue #1 cover copy: "This is Not Your Father's Autobot." #2: "Fort Max Gets the Ax." #3: "Killing Frenzy." The characters would also kill without hesitation and use guns that weren't their signature weapons.
Here's one way to kill the party: Turn cheerful, bouncy Robbie Baldwin from the playfully heroic Speedball into an apparent murderer with a guilt complex worthy of Angel. Now he calls himself Penance, and wears a suit with 612 built-in points of pain, one for each person killed that day. His new powers can only manifest when he is in pain.
In Thunderbolts, however, Penance has come to terms with the Stamford incident not being his fault. He reveals to Nitro the real reason for the suit.The suit wasn't for Robbie, although his survivor's guilt led him to wear it as a form of cutting, it was for Nitro. Robbie captured Nitro in Latveria to punish him for the Stamford incident, put him in the suit and proceeded to beat the CRAP out of him, after which he removes the last spike from his own chest to symbolize that he's freed himself of guilt.
He later returned to the Speedball identity as an instructor at the Avengers Academy, but retained his more serious demeanor. He left the school after finally coming to terms with the Stamford incident, and has since appeared in Nova with his previous cheerful personality restored. He still occasionally uses the Penance helmet though, as it's apparently the only way he can access his pain-based powers.
Much of Marvel's Ultimate Universe runs in this vein. A stunning amount of the process of its "updating" traditional Marvel characters for the modern era has involved inflating the sex and violence content (e.g. the Hulk isn't merely violent or even murderous, but is also cannibalistic; Quicksilver isn't just very protective of his sister the Scarlet Witch, but is in a sexual relationship with her; Tony Stark is a genius as expected — due to a painful cancer-like affliction which has spread brain matter throughout his body and will soon kill him). "Updating" personalities means turning everyone into a complete and utter Jerkass. Spider-Man largely escaped, but the Avengers and X-Men were all turned into such vile bastards that... well... they wouldn't exactly look out of place on the new Battlestar Galactica.
The biggest example of this in the Ultimate Universe is Captain America, who in the 616-universe is the embodiment of American ideals and values, including but not limited to equality, openness to political discourse, and dedication to international harmony. Ultimate Cap is a sexist hardliner who calls the French cowards. His characterization is more of a Deconstruction of the original idea: a man who's been frozen in ice since the 1940s and yet has to be the quintessential American hero for today, despite being the hero of (and thus holding ideals from) yesterday.
Said Ultimate Universe spread to the 616-universe, as far as evil Iron Man and Reed Richards and Cyclops expelling Xavier from the X-Men (even though Cyclops utterly bombed as Top Guy at the school as far as Xavier saving the X-Men's asses during the Messiah Complex X-Over) and starting his own murder squad, a move even Wolverine found distasteful and only agreed to lead to try and keep Scott from turning Wolfsbane, X-23, and Warpath into soulless murderers). Plus the whole "Spider-Man selling his soul" crap.
The whole "Professor X is no better than Magneto" creep from the Ultimate to the main universe that was exemplified by Deadly Genesis, where it was revealed that Professor X led a team of X-Men to their deaths in rescuing his original team from Krakoa and just mind-wiped everyone into forgetting that it happened and trying again with another new team. And that Professor X later realized that the Danger Room was becoming sentient, but ignored it, leading to Danger being created years later.
This all came to a head with the finale of Avengers Vs X-Men, where Cyclops snapped and killed the Professor while possessed by the Phoenix Force. Now he's on the run with his own team of outlaw X-Men, and has already clashed with the Avengers and his former friends. Fittingly, he and Emma Frost have ditched their old costumes for black leather duds.
The ultimate irony is that now Wolverine is the more passive of the two. He's disbanded the X-Force and now runs the Jean Grey School for High Learning, where he trains the next generation of mutants. He's also being depicted as a more traditional superhero in Uncanny Avengers.
A 2004 Thundercats mini, Thundercats: The Return. Lion-O gets trapped in the Book of Omens for five years, and when he gets out he finds the Thundercats beaten, Bengali killed and enslaved by Mumm-Ra. Like Wilykit and Wilykat. Let's just say that puberty has been good to them, and that Mumm-Ra has the same tailor for his slaves as Jabba the Hutt. There is also implied rape of Cheetara by the Mutants. And then there's Lion-O brutally breaking the neck of an ape mutant.
The DCU's Post Crisis universe was so grim, it supposedly drove the Silver Age-inspired Superboy-Prime crazy — causing him to become a mass-murdering fanatic and perhaps the darkest and edgiest DC character of all time.
One of the flashpoint events leading to this was Wonder Woman's killing of Big Bad Maxwell Lord.
The Superboy-Prime saga, which climaxed in Infinite Crisis, was followed by an even darker and edger storyline called 52, and also saw the relaunching of numerous series with a generally darker tone. A prime example is Checkmate; issue #1 featured a team of superpowered spies infiltrating a Kobra base and leaving no survivors (with the Bad Ass heroine of the series, Sasha Bordeaux, shooting the Kobra Big Bad dead, execution style). The series muted its violence considerably after the first half-dozen issues.
While not generally darker and edgier as a whole, the New 52 titles are divided into groups, such as "Batman", "Superman", "Justice League", etc. Two of the groups are known as "The Dark" (supernatural titles) and "The Edge" (titles about anti-heroes).
Blue Beetle was originally a fun book that didn't take itself too seriously - for example, the scarab was played as a Heroic Comedic Sociopath. In the New 52, it was initially just a sociopath, and Jaime couldn't rein it in as much as he used to at first.
There is also a hint of deconstruction. Deadpool grows increasingly disgusted and angered over the dark actions the team takes over the course of the series. You know things are getting bad when Deadpool is having to be the Only Sane Man/moral compass.
Dare, a 1991 take on Frank Hampson's iconic British 1950s space explorer Dan Dare. The 1991 version was written for Toxic magazine by Grant Morrison, and illustrated by Rian Hughes. Dare awakes in the 1990s to find that Britain has become a capitalist society, and that a thinly-disguised parody of Margaret Thatcher has sold Britain to the evil Mekon. During the course of the story all of the main characters are killed - Digby even has his arm blown off - and the final edition ends with Dare blowing up London with a nuclear bomb.
Parodied extensively in the Belgian comic De Kiekeboes, where in one issue, The Simstones, a character from the comic buys the publishing rights to the comic (very meta) and introduces a darker and edgier style.
Marvel Year In Review 1993 parodied this in their own titles, by taking characters that this had been done for, and then making new characters that turned it Up to Eleven:
Spider-Man — Venom — Carnage — Bile (Cannibalistic madman with the proportionate strength of a spider)
Captain America — U.S. Agent — The Patriot Missile ("Blow all them foreigners to hell and let God sort 'em out!")
Thor — Thunderstrike — Godhead (Convinced he is God. Holed up in his compound, waiting for Ragnarok)
New X-Men: Academy X. After House of M, the title was hit by Darker and Edgier hard, but the change was especially marked in contrast with the first half of the series. Under Weir and De Filippis, the book was fairly light-hearted fluff that focused on relationship drama. When Kyle and Yost took over, dozens of students were immediately blown up, and everyone else was left traumatized by their failed rescue attempts. Then a main character was shot in the head and killed. And another main character betrayed the team, was mutilated, and died. They were replaced by a former assassin Tyke Bomb. Succeeding plotlines saw the entire team sent to HELL, one of them tortured and spending a lot of time crying herself to sleep, and so forth and so forth. In fact, most of Kyle and Yost's work falls under this trope. See also: X-Force, mentioned above.
DC's Vertigo imprint revolves around material intended for mature audiences. After the success of Swamp Thing, Doom Patrol, Hellblazer, Shade, the Changing Man, and Animal Man, all of which starred fairly obscure characters from established DC canon, there were a few misfires - up to and including a brooding, psychological take on Brother Power, the Geek. For the uninitiated, Brother Power is a human-sized hippie rag doll given life and super strength by magic sunshine who once ran for a U.S. congressional seat and was last seen orbiting the Earth. Someone tried to make that serious.
Similarly, Grant Morrison himself tried to revive Kid Eternity in a darker and edgier fashion. Kid Eternity was a demi-angel who could summon the spirits of dead famous people. All told, it actually worked out surprisingly well; the miniseries sparked a (short-lived, but still) ongoing by Ann Nocenti, if that's any indication.
Archie Comics' Sonic the Hedgehog seems to be becoming this, as Dr. Eggman very nearly killed off a Freedom Fighter (with a REAL machine gun turret). If it weren't for Sonic going back in time 10 seconds and saving them, almost all of the Freedom Fighters don't seem to be doing any better in the comic's latest issues.
Way back when the comic was humor-oriented, the cover for issue #4 parodied this by promising an "all-new, darker, grittier" Sonic. Turned out he was just covered in dark grit from cleaning the chimney.
Parodied in issue #10 of the old Chip 'n Dale Rescue Rangers comic book. In it, the Rangers are brought to the set of a movie featuring a dark-and-gritty version of a superhero squirrel puppet who originally appeared in a Sid And Marty Krofft-type children's show. The character's creator is shown working as the movie's creative consultant and is not at all happy with the way the movie portrays his creation.
The above story wasn't even the only Archie comic to feature face-melting action. From 1972 to 1974, Archie published a Sabrina the Teenage Witch spinoff, Chilling Adventures in Sorcery as Told By Sabrina. It had the odd combination of straight-up horror stories with art in the familiar Archie house style. One story in particular stands out, featuring a boy who teases a stutterer at school. The kindly teacher happens to be a witch, and gives him an enchanted book that melts his face off, and possibly kills him! The story probably violated several rules under The Comics Code, but somehow gained the CCA seal of approval (perhaps because Archie ran the CCA?)
Paperinik New Adventures is a rare case of this done well. Those stories are way darker than the ones on "Topolino" (the Italian magazine where it is usually published): Paperinik stops fighting the Beagle Boys to defend the Earth from aliens, time travelers and crazy AIs, creating a new roost of supporting cast and using weapons which are much more powerful. However, he remains a very optimistic hero, and the comic gives us several funny and heartwarming moments to balance the mood.
Godzilla: Kingdom of Monsters is this to the entire Godzilla franchise. How dark is it? Godzilla reduces Japan to rubble in the first two issues. The rest of the series has the monsters tearing apart civilization and bringing out the worst of humanity.
However this actually brings it closer to the tone of both the original film and the Heisei era. One of the complaints people had about the series was that, even with that knowledge in mind, it was little too much of a tone shift. Especially in reference to the scene of Rodan eating a child alive. There's another dark scene where Godzilla lets loose his atomic ray on a bunch of people trying to escape Los Angeles which he was currently destroying at that time.
Superman: Earth One was explicitly advertised as being darker, sexier, and moodier, and many standard elements of Superman's story are given a darker spin — for example, Jonathan and Martha are forced to keep Kal-El a secret after government agents secretly impound his spaceship in a secret base.
IDW's My Little Pony: Friendship is Magic comic series is noticably darker than the animated show it was based on. Issue #3 starts with Queen Chrysalis (who by herself is portrayed much more sinisterly here) and her changelings invading a little town of cute loving kitties and sucking all the love out of them. One month later the whole land is converted into the new changeling kingdom.
Image Comics' March 1993 one-shot Darker Image is this, featuring the first appearances of Dark Age of Supernames heroes Bloodwulf and Deathblow. It is also notable for containing one of the first appearances of The Maxx.
Before this were a few twists on dark storytelling in the series; Asterix in Switzerland's plot involves the heroes' efforts to save an innocent from murder. Quaestor Vexatius Sinusitus' potential death offered a jarring but refreshing sense of drama to the otherwise frivolous comedy strip. The same story also contains a more serious look at the Romans than usual - normally, Asterix villains tend to be Punch Clock Villains, Ineffectual Sympathetic Villains or just ordinary people who happen to get in the heroes' way (occasionally even Designated Villains, Played for Laughs), but Varius Flavus's actions (corruption, insane decadance and poisoning his opponents) are much more like what evil Roman patricians in history actually did. Oh, yeah, and an actual Roman orgy is depicted.
Stories featuring similar moments of deadly menace include Asterix and Son, where the village is decimated, and the impending threat of Orinjade's execution in Asterix and the Magic Carpet. Also Obelix All At Sea, in which both Asterix and Obelix almost die (and the villain does, breaking Nobody Can Die), and Asterix and the Picts, which involves Scarpia Ultimatum and a much more complicated plot than usual. To a lesser extent, The Roman Agent and Caesar's Gift are both about just how ridiculously awful living in their Quirky Town would be.
In Uderzo croqué par ses amis, a compilation album of short stories drawn by various artists about Uderzo, one story is a realistically-drawn, historically-accurate, painfully serious take on the concept of a pair of Gaulish warriors fighting Romans using magic potion. For instance, the magic potion appears to be a kind of religious Magic Feather, they put the skulls of dead Roman soldiers around their village to keep them out (like the historical Gauls did), and they murder Romans with swords. It turns the usually ridiculous little Gauls into something quite dramatic and mystical and Bad Ass.
Uderzo croqué par ses amis also has another story in a similarly realistic art style, but with the usual characterisations of the Gauls. The story contains a gag where Asterix and Obelix accidentally catch Vitalstatistix in flagrante delicto with a hot blond who is not his wife, which is depicted in intentionallySquicky detail (since Vitalstatistix is both a beloved childhood character and a fat, ugly middle-aged man). Asterix is not exactly sexless but a gag like that would never get into the main stories.