Aquaman had the death of Aquababy, Arthur Jr., in the Death of a Prince storyline in Adventure Comics #452. It changed the tone of the stories completely and had ramifications for decades as Aquaman dealt with grief and loss, his marriage fell apart and continued to have ups and downs as Mera blamed him for the child's death, Aqualad's relationship with Arthur was strained for a period, and Black Manta became cemented as his arch-nemesis for killing his son.
A Death in the Family have been this for the entire Batman lore, so much that said Wham (the death of Jason Todd) overshadowed easily the rest of the story arc.
Batman Beyond: The 2.0 series is full of those. In the Issue #1, who establishs the comic-book as a set-up into a one-year Time Skip, Dick Grayson is the new Mission Control for Terry McGinnis and, also, the latter and Bruce Wayne had a fall-out. The specifics are finally revealed as of Issue #25. Issue #26 shows Terry confronting the killer of his father and giving him a beatdown. The real whammer is the fact that, when discovering a photo of his father in the apartment of the killer and asking for answers, Bruce tells him to back off, implying that he knows something. Issues #28 & #29 happens after Return of the Joker and reveal what drove Dick to never speak to Bruce ever again. Finally, a more minor one happens in Issue #30 when Jake Chill die from Ghoul's Joker Toxin and it's shown that Terry's antidote doesn't work on the new strain.
In the epilogue of the Issue #5 of Batman Beyond (Rebirth), Terminal is assaulted by the janitor for insulting the Joker and misusing his name...then he rephrases, stating no one misuses his name, as he pulls his hat off and reveals that he is the Joker!
Batman: Curse of the White Knight: Oooh boy. Quite a few. In Issue #2, Joker crashes Jim Gordon's mayoral campaign and outs his daughter as Batgirl to the public, Batman discovers that Harley is pregnant, and Azrael has the Mad Hatter hack the Batcave, completely destroying it and Wayne Manor. In the very next issue, Azrael kills Gordon and Harley goes into labor. In Issue #5, Azrael (now in his own Batsuit) walks into Arkham and slaughters all of the inmates. In Issue #6, it is revealed that the true ancestor of Bruce Wayne is Bakkar, who killed Edmond Wayne and stole his identity to rebuild Gotham Valley into Gotham City (meaning that Azrael is a Wayne). Also, Harley shots Joker, killing him. Finally, in Issue #7, Bruce unmasks himself and announces that he's dissolving his company and giving away his entire fortune to help Gotham. Then he goes after Azrael, apparently prepared to kill him.
Batman: Endgame: Two of them. Issue #37, when Batman goes to the hospital where patient zero for Joker's virus is and discovers Joe Chill, the murderer of his parents (meaning Joker knows Bruce' identity), and Issue #40, when Batman and the Joker seemingly commit a Mutual Kill.
Batman Eternal: In Issue #26, Gotham enters martial law, Alfred is taken from his hospital bed by Hush and wakes from his coma to find himself in the care of the Joker's Daughter, and Stephanie now has a very large bounty on her head courtesy of her father's friends, meaning she'll have to keep running for her life. And later, in Issue #34, Wayne Enterprises is taken over by the government, and former Big Bad Hush is revealed to be working for someone else entirely.
Night of the Owls: Both Batman #7 & Nightwing #7 reveal that Dick Grayson was meant to be recruited into the Talons and the Court unleashes all of the Talons on Gotham. Then, in the end of Batman #10, Lincoln March is revealed to have faked his death, is a member of the Court of Owls and alleges that he's a forgotten brother of Bruce. Issue #11 puts a lot of doubt on the claims of being Bruce's brother, as Bruce himself points out.
Issue #1: Barry Allen, recently brought back from the dead, tries to figure how much things had changed. When he learns about the villain morgue and the hero cemeteries, he asks Hal who else had died since he was gone. Hal uses his ring to show everyone. Flash even sits down in sorrow when he sees Ralph & Sue Dibny among the dead.
Issue #5: The White Light only serves to make Nekron stronger, and he sends a Black ring to cause "Bruce Wayne" to rise, and 10 black rings are sent out, which then place themselves on the fingers of Superman, Wonder Woman, Green Arrow, Ice, Animal Man, Donna Troy, Kid Flash, and Superboy, turning them into Black Lanterns, with Hal Jordan and Barry Allen being pursued by the remaining two.
Brightest Day: It is revealed in the issue #60 of Green Lantern that Krona, an Immortal responsible (even if inadvertantly) for the creation of the Multiverse, the birth of the Anti-Monitor and who was one of Nekron' allies, is the enemy who wants to steal the Entities. Oh, and also, he forced himself to evolve into the same type of being as the Guardians, with the same powers.
And then, the entities (loyal to Krona from his years of taking care of them) possess the bodies of all of the remaining Guardians of the Universe, save for Parallax, who re-enters the Main Green Lantern Power Battery and takes control over 99% of the Green Lantern Corps.
Crisis on Infinite Earths, the single biggest wham episode in all of comics history. Was the ultimateNothing Is the Same Anymore for the DC universe. No event has topped it since, though DC has had several since that range in reception from great to good to average to suckish.
Issue #2: Batman travels to an ancient tomb in order to use Baby Darkseid's Omega Beams to go on a one-way trip back in time to fight Barbatos. However, the place turns out to be a trap laid by the Court of Owls, who use a metal called Batmanium to transform Batman into a doorway and allow Barbatos and the Dark Knights to enter the DC Universe.
Issue #4: Dream appears to explain the origin of the Multiverse, Barbatos's identity, and send Superman and Batman to the World Forge. THEN they find it's already darkened and guarded by a giant corrupted Hawkman.
In the one-shot The Wild Hunt, we see Red Death's Heel–Face Turn, the Dark Knights's Hazy-Feel Turn and The Reveal of Universe 53, home to a Super Ape version of the Justice League.
The Issue #2 of Dark Nights: Death Metal reveals that not only has the Batman Who Laughs (who have been killed by Wonder Woman shortly before) been revived, he transferred his body to a Bruce Wayne/Dr. Manhattan, becoming an abomination he calls The Darkest Knight.
In Justice League #45, Mobius (aka the Anti-Monitor) kills Darkseid by using the Black Racer combined with Anti-Life, and thereby rattles the cosmos.
Justice League #50 is full of whams, but the cherry on top is Grail killing Superwoman and using her newborn child—son of Mazahs—to absorb the Anti-Life Equation, all of the powers from the Justice League gods (sans Batman), and initiating an ancient Amazonian ritual to resurrect Darkseid through Superwoman's child. Whom Grail can control, thanks to the Anti-Life Equation.
Issue #6 of the DC Infinite Frontier miniseries is a big whammer, especially the ending: Barry Allen crashes into the remains of Multiverse-2, first mentioned during The Multiversity and now clarified to be the remains of the Multiverse the Anti-Monitor destroyed in the original Crisis. Here, Barry encounters Pariah who demonstrates the power of creating brand new worlds tailored to individuals. Barry's sent to an Earth called "Earth-Flash.1" where his mother is alive and he has a Flash Family consisting of Iris, Wally as Kid Flash, Jay Garrick as Flash, a younger version of Wallace West, and younger versions of Don and Dawn (the Tornado Twins). Pariah ominously implies he plans to do this to everyone. Meanwhile, Darkseid notes that the energy that created Earth Omega was the Great Darkness.
DC Universe: Rebirth #1, which kickstarts the DC Rebirth event, nails Wham Moment after Wham Moment as The New 52 is flipped on its head: Wally West returns to continuity, there have been three Jokers, the Justice Society and the Legion of Super-Heroes are still around but locked away memory-wise and — the big kicker — the creator of Prime Earth and the cause of the loss of ten years is not Barry Allen's fault, but the fault of Doctor Manhattan.
Issue #1: The series continues several years after the end of the original Watchmen. Ozymandias's plan failed, New York and possibly the entire world is in chaos, Rorschach has a successor, Ozymandias himself has gotten cancer after infecting Dr. Manhattan's many friends and enemies, and now Ozymandias and Rorschach II are presumably to travel to the DCU in order to find Manhattan and plead for his help in fixing their world.
Issue #2: Nuclear war finally breaks out on Earth Watchmen, and the same appears to be on the horizon for Earth DC, thanks to Russian and Markovian reaction to conspiracy theories about the true origins of the metahuman community. After making it to Earth DC with Rorschach, the Mime, and the Marionette, Ozymandias tries to get an audience with Lex Luthor only to find out that the Comedian is alive.
Issue #3: It's revealed that Dr. Manhattan actually saved the Comedian and sent him to the DC Universe, the Mime may actually be a metahuman and Batman locks Rorschach in Arkham Asylum, thinking him insane.
Issue #4: Rorschach II is revealed as Dr. Malcolm Long's son, as is the fate of Byron Lewis. Alfred tries to convince Batman that locking Rorschach up was a mistake, and that they need to find out what he knows. Rorschach and Saturn Girl team up to escape Arkham.
Issue #5: The Supermen Theory leads to several countries completely closing their borders shortly before Black Adam breaks international law to rescue Jack Ryder and declares Kandaqh a sanctuary for metahumans, leading to a severe increase in global tensions. Batman tracks down Ozymandias and ultimately winds up beaten by rioters before getting dragged to the Joker. Lois meets with Lex believing him responsible for the Supermen Theory, but he claims not only to be innocent this time but also looking into who is behind it and reveals that the guilty party was once a member of the Justice League. Rorschach II and Saturn Girl track down Johnny Thunder, who has found the lantern of Alan Scott.
Issue #6: Joker takes Marionette, Mime, and a captive Batman to an underground villain meeting. The villain community is terrified at the prospect of the upcoming war. Wonder Woman is apparently in hiding. Captain Boomerang is at the meeting: it turns out that the Suicide Squad was sent to Kandhaq to assassinate Black Adam, they failed, and only Boomer escaped. The Comedian appears and starts shooting everyone. Joker, Marionette, and Mime escape and form a partnership of their own to find Doctor Manhattan, with Joker apparently forgiving them for killing his henchmen in earlier issues. Also, parts of the Supermen Theory are confirmed via Typhoon's DEO profile.
Issue #7: Adrian has been lying the entire time about his cancer, all as part of a sick plan to manipulate Rorschach II into becoming his servant. He steals the golden age Green Lantern's lantern and uses it with baby Bubastis to forcibly summon Dr. Manhattan, who essentially tells Veidt to get bent because he's busy with other things, while confirming to the reader that Manhattan's manipulations of the timeline extend to sabotaging the incident that would have made Alan Scott Green Lantern, killing him instead. Marionette meanwhile finds out that she's pregnant from Manhattan, with the child she's carrying and not her first born being the reason he spared her. Oh, and Manhattan informs Veidt that in one month's time a disaster will occur of such magnitude that there is evidently no longer a future for him to witness. Even worse, Saturn Girl's trip to the past is explicitly to stop someone from changing the future via killing Superman and (along with Johnny Thunder) she is brutally attacked and taken hostage by Adrian, who has decided he has enough information at hand in order to take advantage of the situation and 'save' both his world and DC's, while Marionette and Mime are on the loose with a bound Comedian and the lantern to celebrate their pregnancy. And as Manhattan leaves for Mars once more, we learn the disaster in one month's time he can't see past is in the form of a catastrophic battle between him and Superman, somehow left hopeless and seemingly dangerously out of control. Which according to Manhattan means one of two things: either Superman somehow kills him...or Jon kills everything.
Issue #8: Things are reaching a boiling point as Firestorm is involved in a fight with various other Russian heroes, leading to a group of protesters being transformed into glass. Superman tries to defuse the situation by talking to Firestorm and, despite Batman trying to warn him not to pick a side, ends up siding with the Metahumans, sparking another incident. Even worse, Ozymandias, using the Manhattan-esque radiation, creates a huge explosion in the area, in an attempt to frame Manhattan as the culprit. As well, Lois ends up obtaining a USB Drive with a video showcasing the Justice Society of America... and she has no idea who they are.
Issue #9: Dozens of heroes go to face down Dr. Manhattan following the incident in Moscow. Their complete and utter misunderstanding of his situation leads to a Curb-Stomp Battle in Manhattan's favor. During this time, Manhattan shows that Firestorm was created, not by accident, but on purpose by Martin Stein, who sought to groom him as a surrogate son. As well, Batman believes that Manhattan's actions are being directed by another and Lex Luthor reveals he was the one who sent Lois the video clip of the Justice Society and that they're connected to the other missing heroes such as Wally West. At the end, as Wonder Woman seeks to bring about peace between the human and metahuman communities, Black Adam and his own metahuman army invade.
Issue #10: While the heroes on Mars are revealed to be not dead, we also learn that Dr. Manhattan has repeatedly observed the DC Universe's Comic-Book Time constantly pushing Superman's history around. His creation of the New 52 era (as a result of Manhattan causing the death of Alan Scott, and implicitly having caused the deaths of Jonathan and Martha Kent) was because he couldn't understand why Superman was the Hope Bringer and, upon creating the moody Darker and Edgier Superman of that era, accidentally created a Superman who would either kill him or cause Manhattan to wipe out the DC Multiverse.
Issue #11: As Batman and Wonder Woman fruitlessly try to fight back against a raging world, Superman awakens. Luthor reveals to Lois that he's been collecting strange variants of all the same photos, one of which was found at the sight of Wally West's return and fuels his belief that he and Superman are intricately locked in a hopeless stalemate. Ozymandias reveals to Saturn Girl that he was behind the reveal of the Superman Theory, the deaths in Moscow and luring the metahumans to Mars, all so he can destroy this world's faith in the metahumans, swoop in and save the world. He, then, promptly makes Saturn Girl disappear from history by realizing Superman never remembered her. As Superman comes to confront Black Adam and his entourage, the fight crashes him near Dr. Manhattan, leading to the fated fight...
Issue #12: Superman is confused as to why Dr. Manhattan isn't helping him against the international coalition while Batman and Alfred convinced Reggie to turn Roscharch into a force of good. The preordained fight ended up being Superman pummeling Pohviz from behind, with the Man of Steel inspiring Manhattan to do the right thing. Manhattan saves Alan Scott this time, allowing Clark to become Superboy once more and officially bringing back the Legion of Super-Heroes and the Justice Society of America into continuity. Manhattan realizes that Superman is the Hope Bringer, bridging the gap between heroes old and new. He gathers the other figures of the Watchmen-Earth and Ozymandias reveals it was his plan all along for Superman and Manhattan to meet so he can reinspire him to save their world, though when the Comedian shoots Ozy, Luthor arrives to send him back to his point of death in retaliation for being shot at. Manhattan leaves Marionette and Mime on Earth-Prime to serve as a bridge between the two Earth. On the Watchmen-Earth, New York is still destroyed, but Manhattan did away with all the nuclear weapons, forcing actual peace talks and, inspired by Superman's actions, Manhattan sends Mime and Marionette's son — Clark — to Dan and Laura. On Earth-Prime, Manhattan apologizes to Carver Coleman for being a terrible friend and convinces him to come out, ultimately becoming a beloved LGBTQ+ symbol. Alan Scott promises to reveal the truth behind the Superman Theory, leading to Martin Stein being arrested and talks of impeachment towards the president for his actions while Wonder Woman urges for the recreation of the Global Guardians. While the future seems uncertain, Lois and Clark go to spend time with a still-alive Jonathan and Martha Kent.
Even relative to the overall aura of Total Mind Screw surrounding Grant Morrison's run of Doom Patrol, the Candlemaker arc was made of WHAM. A particular standout is Issue #57, in which the Chief explains that he was responsible for the entire original Patrol's tragic backstories and has been controlling every major plot beat they've gone through since. He is then decapitated by the Candlemaker, who then goes on to destroy Cliff's brain.
Earth 2: Issue #1 starts the series with a bang, given the deaths of Superman, Batman, and Wonder Woman.
And then, Issue #16 shows Brutaal killing Steppenwolf. He's revealed to be Superman.
Flashpoint: In the Issue #2 of Batman: Knight of Vengeance, Selina Kyle is revealed to be a quadriplegic Oracle, Jim Gordon accidentally shoots one of the Dent kids, and he himself gets killed by the Joker, who turns out to be Martha Wayne.
Issue #1: The Justice Leaguers are missing, the power grids in cities are down, The Crime Syndicate has freed and gathered all the super-villains to take over the world, and Nightwing's secret identity is publicly revealed. And that's all in the first issue.
Issue #4: Sinestro returns to save Batman from Power Ring, after seemingly leaving the universe to its fate after Wrath of the First Lantern.
Issue #5: Sinestro kills Power Ring, leaving his ring to seek out others. The now-freed Ring of Volthoom sends out a multiversal pulse to bring in the entity that destroyed Earth 3 to Prime Earth.
Issue #6: The man with the hood is Earth 3's Lex Luthor, who calls upon the powers of Mazahs and is set on destroying both sides of the conflict to become Prime Earth's greatest hero.
The final issue is also quite whammy: Luthor's team saves the Justice League and kills the Earth 3 Luthor, Superwoman's child is revealed to be that of Earth 3 Luthor's and Bizarro ends up dying fighting the Earth 3 Luthor. At the end, Vibe and Elemental Woman disappear, Luthor and Captain Cold accept a presidential pardon for their crimes for their actions, Luthor realizes Bruce Wayne is Batman... oh, and while Superman thinks that Darkseid is the one who destroyed Earth 3, it isn't — it's the Anti-Monitor and he's gunning for Darkseid once he's strong enough!
In Batman #680, along with the Face–Heel Turn, Batman actually unmasks and reveals his secret identity to The Joker, of all people. Joker doesn't seem to care about this and then starts indirectly addressing the audience.
Batman #701-702: After resting for three days after his confrontation with Hurt in Batman R.I.P., Bruce finally ventures into the Room Full of Crazy hidden in Wayne Manor, with the names "Barbatos" and "Thomas" scribbled all over the walls.
It appears that Darkseid didn't just used the Omega Sanction on Bruce. After hitting Bruce with the Omega Sanction, Darkseid pulls out the Ancestor Box which, as the box itself puts it: "It learns! It knows! It bonds! It lives to become the fate you can't escape! Omega is death that gives and gives forever. Omega ‘tailor makes’ an unbeatable ‘life trap’ just for you. It uses 'history’ to do it!”
In Batman R.I.P., we learn, aside from the most important elements about R.I.P. itself and the character of Simon Hurt, that the narrative of this arc is closely tied to The Return of Bruce Wayne.
Batman Inc. (New 52) have three of them:
#6: Knight gets his neck snapped by Talia's Elite Mook The Heretic, and Batman is captured by Leviathan.
#7: The Heretic is actually Damian's clone, shown at the end of Batman and Robin #12.
#8: The Heretic brutally murders Damian on Talia's orders, although later is revealed that she didn't order Damian's death. It was the impulse of her large, unstable Damian clone and her Leviathan goons.
Issue #2: At the end, Lois Lane receive Arsenal's confession tape in an e-mail (despite the Sanctuary confessionals supposedly being immediately deleted), with the promise that more will be sent soon. This is after it is revealed that Superman AND Batman trusted the safety of the system enough to reveal their worst fears and secret identities to the AI, meaning that the mysterious Puddlers now know who they are and how to best hurt them.
Issue #5: The Wally West who was found dead at Sanctuary is five days older than he should have been, confirming that some form of time travel is involved.
Issue #8: Wally West was responsible for everything that happened at Sanctuary. He's also the one who's been rebuilding the confessionals and sending them to Lois Lane.
The culmination of the infamous The Judas Contract arc of Teen Titans. Terra, who saved them from Slade and has fought beside them for ages, is The Mole. She's always been The Mole. She's Slade's weapon and lover, and she's spent her entire time setting them up.
The 1969 Justice League of America comic issue #71 "And So My World Ends..." was this for the Martian Manhunter, killing off his planet, most of his entire species, driving him out of the Justice League, and setting the tone for every story featuring J'onn J'onnz to be published after the fact.
The Power of Shazam issue #38, where Mister Mind escapes from custody and uses Mister Atom's nuclear core to blow up Fairfield, killing several supporting cast members, including Mary Marvel's parents (although it turns out they survived). Not just for the comic itself, but for the entire Captain Marvel's franchise; Captain Marvel's adventures are typically Lighter and Softer than the other DC heroes' stories, so for something like this to happen was so unexpected and stunning that it changed the course of the series. It also changed reader-perception of Mister Mind forever, changing him from a Silver Age relic to Captain Marvel's most dangerous and personal enemy.
In Issue #6, Kenan's father seemingly dies stopping Human Firecracker, only for the last page to reveal Dr. Omen has his body in stasis... and Dr. Omen is implied to be Kenan's supposedly dead mother.
The last page of Issue #8 reveals an incredibly unexpected character... Ching Lung, the Yellow Peril Chinese villain from the cover of the first ever issue of Detective Comics!
Issue #10 has Ching Lung reveal who he actually is. He's I-Ching. Or, as it turns out, I-Ching's evil(?) twin.
Superman Reborn was one itself revealing that the pre-Flashpoint Superman was actually split in two and one of the halves is, in fact, the New 52 Superman. Also, the person who seems to be at first the New 52 Clark Kent and pretends that the New 52 Superman took his identity and is a completely separate person from him is, in fact, Mr. Mxyzptlk (who disguised and gave himself false memories to get away from Mr. Oz).
The Oz Effect is this for the entire Superman saga in DC Rebirth, especially when it's revealed that the antagonist Mr. Oz is, in fact, Jor-El.
The soul-crushingly bleak Supergirl (2011) #23 has Kara completely disintegrated (in a particularly traumatic way) by the amnesiac Cyborg Superman who uses her DNA to reconstitute himself into an organic being. He immediately discovers he is really Zor-El and has just killed his daughter. To top everything off a very angry Brainiac (who 'created' Cyborg Superman) has just arrived.
