Marvel Comics managed to do this with the very first issue of Thunderbolts. The Reveal is considered one of the best in the industry. In case you're wondering: The Thunderbolts were a team who first showed up in Incredible Hulk after the whole Onslaught thing (long story short, the Fantastic Four and The Avengers are missing and no one knows how the rest of the superhero community can fill the vacuum), and made various miscellaneous appearances. They got their own book and their public debut by restoring order to the ruined Manhattan, defeat the Wrecking Crew and receive public approval even greater than even the Avengers themselves had after went public. And then we get to see Citizen V, the team leader, without his mask. And, perhaps more significantly, we get to see him with his real mask. The Thunderbolts are the Masters of Evil, the nemeses to the Avengers, led by Baron Zemo. Holy shit.
What really makes it impressive is how Marvel kept it under wraps. There was no idea that there even was a secret to give away; Peter David even had the solicitation rewritten for the team's first appearance in Incredible Hulk to keep it quiet.
They even went so far as to make the early appearances of the team and bulk of the first comic slightly dull and generic, fully intending for The Reveal to make up for the likely poor initial sales.
The Fantastic Four changed its entire tone and "atmosphere" after Galactus made his first appearance. Then on, the stories and art both became "bigger". Instead of helping roust escaped inmates or reel in a bank robber, the Four were confronted by whole unknown races like the Kree and the Inhumans. Where Doom before just attacked the FF, he attacked the entire world after absorbing the power of the Surfer. Weapons got gigantic, monsters and robots were more realistic, and the drawing became more three-dimensional with Kirby Energy Spheres heaped on in abundance.
Blame the Comics Code Authority - "If a major character lacking power and unconnected to the antagonist dies in the issue (back then) the death must be listed on the cover." - listing a whole BUNCH of people was an awesome cover for it. It's also universally regarded as the single GREATEST Wham moment in comicdom. Ever.
Spidey has his share of whams. In The Clone Saga it's death of Aunt May. First issue of the JMS run does another wham with Ezekiel asking a simple question - what if Peter's powers don't come from radioactivity? — and another one when Aunt May finds out he is Spider-Man. Brand New Day does that with the beginning of The Gauntlet arc — Electro destroys the Daily Bugle.
Amazing Spider-Man #698 gives us perhaps one of the biggest whams yet. We learn that somehow, Doctor Octopus has switched bodies with Peter. Keep in mind, that Doc Ock is at near-death and that he's essentially doomed Spidey to an undignified death, while he gets to run rampant with all of Peter's powers, abilities, and memories.
Superior Spider-Man #9 has Doc Ock wiping Peter from his mind and finally taking full control of Peter's body.
Superior Spider-Man #25, Peter DOES come back, though he's playing smarter than last time.
In an issue of Ultimate Spider-Man, Marvel uses one of the most infamous events of Spider-Man to pull a massive wham episode. They parallel the Death of Gwen Stacy exactly with the minor detail of switching her out for Mary Jane Watson, the visual is so exact, even down to the way she bends as she falls, that when her head snaps back as Spidey's webline hits her, no reader of the original comic is left with even the slightest doubt that they just killed off Peter's most recognizable love interest, the flame headed Mary Jane (but they didn't).
The Ultimatum event was a big WHAM for the entire Ultimate Marvel line. We knew that a huge disaster would prompt a line-wide Re Tool, but we didn't know how bad it would get. Half the X-Men died. B-list heroes (in this 'verse) Daredevil and Dr. Strange died. Spider-Man... looked like he died. Xavier and Magneto died. Dr. Doom died. Wolverine, whose mainstream version is patron saint of From a Single Cell, died. And the Fantastic Four and surviving X-Men disbanded.
Then in #600, it's revealed that Cap isn't quite dead and that there's a way to save him, leading into the Captain America: Reborn miniseries.
The Walking Dead, issues 45-50. 9/10's of the cast die, including several main characters and a newborn baby. The few that survive are scattered by the Governor's attack, and forced out of the relative safety of the prison they had been using as a home.
Issue 83 features another massacre: Morgan, Jessie, Ron, and Douglas are devoured, and Carl is shot in the eye.
Issue 100 features the death of Glenn, leaving only 4/15 of the original cast still standing.
The concluding issue of the War in the SunStory Arc in Preacher, the gravity of which was ominously foreshadowed in an early conversation with the Duke:
Duke: Ya know ya got a... hard time aheada ya, don't ya? Jesse: I know somethin's comin'. I got a feelin' like... I dunno. Like nothin's ever going to be the same again. How bad you figure it'll get? Duke:As bad as it can.
Invincible Issue 11 reveals that Omni-Man's origin from issue 2 is carefully-crafted bullshit. Nolan Grayson is a lie. Omni-Man is a lie. The Viltrumite sent to Earth to gather information for its conquest is the truth — and he wants Mark's help in doing it. When Mark refuses, he beats him half to death and when confronted with his unconditional love ("YOU, Dad. I'd still have you."), flies off into space with no sign that he's ever coming back.
Watchmen: Chapter 3. And arguably 9, and, of course, 11.
The first volume of Runaways ended with Alex revealing himself to be the mole. Immediately after that, he got himself killed. And immediately after that, all twelve members of the Pride got blown up, taking the series in a completely new direction. The end of the second volume was in some ways even more shocking, as a major character who wasn't evil got Killed Off for Real.
Doonesbury is known for throwing emotional curveballs every now and again. Among the most memorable were Dick Davenport's death-by-heart-attack (in the middle of bird-watching, with the Wham panel suggesting that he got the picture he was looking for) and original cast member BD losing his trademark helmet (and his left leg) after getting hit by an insurgent IED.
Don't forget Andy Lippincott contracting then dying of AIDS, which also forces Mark to come out of the closet. Andy's came up out of nowhere during a storyline where Lacy Davenport looked up her former staff who had AIDS, and the extent was shocking. But a few months after Andy's tragic death, his spirit came to Mark and informed him that he was gay. Both Mark and the audience were shocked by this.
Duke's death. While temporary (He was zombified...long story), it was still pretty shocking.
Li'l Abner. The marriage between Abner Yokum and Daisy Mae.
Peanuts had the first major Wham in newspaper comics when Charlie Brown went to the hospital. As the rest of the characters fretted about Charlie's life, around the country, readers sent "get well" cards to him as well.
Blackest Night #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.
Sgt. Fury lost the love of his life, Pamela Hawley, after he had bought the engagement ring and went through hell to retrieve it during a battle. She died during a London air raid.
The culmination of the infamous Judas 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 first time Alan Moore became famous in the US, it was for a one-issue one of these in a comic that was all but canceled; word of mouth alone made it one of the most famous single issues that year. By the time of the Anatomy Lesson, Swamp Thing had spent nearly ten years trying to reverse the accident that transformed him so he could be plain ol' Alec Holland again. He never will be Alec Holland. He isn't Alec Holland. He never was Alec Holland.
His Youngblood: Judgment Day was this for Youngblood - Knightstrike is acussed of killing Riptide, team is disbanded and entire Universe turns out to be plunged into Darker and Edgier setting by magic book. Same with his Supreme - the very first issue is this for the previous series, revealing that Supreme is only one of many versions of Superman-esque character and now reality rewrites itself, giving him new life and new memories.
For those who stuck with the comic after "EndGame", current head writer Ian Flynn likes to do these. At first, it was for "cleaning house" (Killing Tommy Turtle, fusing all the Chaos Emeralds in the universe into a set of seven, etc.) then, came the Destruction of Knothole arc, arguably one of the defining arcs of his run. Robotnik gets dead serious for possibly the first time since the EndGame, sics a FLEET of floating fortresses on the Kingdom of Knothole, destroying it effortlessly, confronts Sonic one on one (again, for the first time since EndGame), and OWNS him. Brutally. For better or worse, THIS is the arc to base your opinion of Ian Flynn around.
And Flynn's had a few other wham episodes since then. The "Enerjak Reborn" arc had Doctor Finitevusbrainwash Knuckles into becoming the new Enerjak, which leads to the destruction of the Dingo Regime, the death of Knuckles' father Locke, and the remnants of the Dark Legion joining with Eggman. And then a little while after that, we get issue 200 - Sonic defeats Eggman so epically, the doctor has a huge Villainous Breakdown, which allows his position as Big Bad to be usurped by the Iron Dominion, who spend the next dozen or so issues curb stomping the heroes all over the place.
Archie Sonic is generally good for a Wham Episode every milestone issue. Once 225 hit, there was a rapid fire of them even:
Issue 225 features the Death Egg, the return of Silver Sonic, and a Cosmic Retcon being activated just as we discover Sally's apparent death via gun turret… Issue 230’s wham was spoiled, but that doesn’t take away the full level of shock that was Sally being roboticized. Finally issue 234 ended in Antoine nearly being Killed Off for Real and left in a life-threatening coma.
Meta Example: The now infamous lawsuit between Archie and the comic’s former head writer Ken Penders ended with a swift removal of anything the writer had created during his tenure at Archie, which was a majority of the non-Sega original cast, and a number of whom were crucial to ongoing story lines at the time. The shift is most clearly seen in the Endangered Species Arc, which suffered huge last minute rewrites and resulted in several major characters and an entire civilization being Put on a Busoff screen.
Issue 250, the first part of the final arc in the WorldsCollide crossover with Mega Man ended with Eggman throwing Dr. Light out of the Wily Egg to fall to his death. Mercifully, Shadow saves him next issue.
The Worlds Collide crossover ended on a hell of a wham in issue #251 with Eggman’s attempt to stop Super Sonic from undoing his Cosmic Retcon resulting in reality literally shattering around them. However the full weight of what happened isn’t shown until #256, when it is revealed that said actions caused the multiverse to collapse, just as the planet shatters from the energy backlash. For perspective: that’s a Class X-5 case of Apocalypse HowIn-Universe, and a Class Z for any characters/events/places that were lost from the Penders Lawsuit.
Johnny, a prominent member of the Freedom Fighters, being killed early in the Sonic Adventure arc really changed things.
Volume 9 of Hellboy, "The Wild Hunt". It starts off looking like a fairly simple "Hellboy beats people up" arc, as the last volume was. By the end, it turns out that Hellboy is the heir to the crown of England.
Volume 5 of Empowered. Willy Pete kills eight and a half super heroes, and blows up the Superhomeys space station. Mind F***, one of the very few decent supers in the world, dies in the process.
The second-to-last issue of Buffy Season Eight. The Seed of Wonder is destroyed, all the magic in the world disappears, and Giles dies.
In the Luna Brothers' The Sword: First when Malia rips out Dara's womb and stomps on it; then after an epic Roaring Road Of Rampage Dara discovers that the whiny tag-along Justin is Phaistos and the whole thing was a plot to get revenge himself after a millennia of realizing what a prick he was, and then he kills himself with the sword; and when Dara throws the sword in the volcano and goes home and dies(?) after ALL her injuries return.
Not content with ending the Democracy arc in Judge Dredd with Dredd's resignation and exile into the Cursed Earth, 2000AD then went on to top it not long afterwards when they revealed who The Dead Man was.
"E For Extinction": The entire course of the franchise changes. A fleet of Sentinels obliterates the island Genosha, killing everyone except for a handful of survivors. Emma Frost (who survived the destruction) realizes that she has a secondary mutation, and joins the X-Men permanently to get revenge on the culprit. Professor Xavier (controlled by Cassandra Nova) outs himself as a mutant on live television.
"Planet X" is a concentrated series of these. Xorn (who was brought onboard with the team 30 issues prior) is revealed to be Magneto in disguise. He subsequently cripples Xavier again and tries to exterminate everyone in New York. Logan and Jean are trapped on an asteroid hurtling into the sun - and he stabs her in the chest, reawakening the Phoenix. A massive battle ensures in New York, which results in the (supposed) deaths of Magneto and Jean.
It's fairly easy to characterize Chris Claremont's famous run on Uncanny X-Men as a prolonged string of these, as he took the series to heights that defined the franchise and changed the face of the industry:
Issue #131: Colossus is forced to kill Moira McTaggert's mutant son, Proteus, when he escapes from a containment cell.
Issue #131: The Hellfire Club shows up for the first time - and captures nearly the entire roster of X-Men - then announces that Jean Grey (Phoenix) has become the club's new Black Queen, while Wolverine is the last man standing and swears to get revenge.
Issue #134: Mastermind manipulates Jean into becoming the Dark Phoenix, an incredibly powerful entity that blows up the X-Jet with the team inside.
Issue #137: The ending of the "Dark Phoenix Saga". The X-Men are forced to fight a group of Shi'ar Imperials in order to win Jean's freedom, and she ends up sacrificing herself to protect the team and stop the fighting. In the next issue, Cyclops quits the team and Kitty Pryde joins the school.
Issue #167: After foiling the Brood's plan to infect the entire team, the X-Men return home — only to discover that the Brood Queen infected Xavier, who hunches over and turns into one as the team looks on horrified. The team manage to save his consciousness and clone a new body for him - which has the added effect of letting him walk again.
Issue #200: Magneto's trial for crimes against humanity ends with him being found not guilty. He subsequently performs a Heel-Face Turn, and is asked by Xavier (who departs for the Shi'ar homeworld) to become the new headmaster for the Xavier Institute.
Issue #201: Along with introducing Madeline Pryor's son, Cable, for the first time, Scott and Storm duel in the Danger Room to see who will lead the X-Men — and Storm wins, leading Cyclops to leave the team again.
Issue #281: Trevor Fitzroy is introduced, and commands his Sentinels to attack the X-Men and Hellions, leading to both Jean Grey and Emma Frost being declared dead by the team.
Issue #303: Colossus' sister (and long-running supporting character), Illyana, dies from the Legacy Virus, and is the first casualty that hits close to home for the team.
Issue #370: Colossus sacrifices himself to synthesize a cure for the Legacy Virus.
The Astounding Wolf-Man: Issue #7, mainly for its ending where Gary's wife is accidentally and brutally murdered by his then-mentor Zechariah. The entire premise of the series changes completely after this issue, focusing on Gary on the run and trying to prove his innocence, while Zechariah has a Heel-Face Turn and becomes a primary antagonist, and Gary's daughter Chloe becomes a Heel-Face Revolving Door for the rest of the series. This is also the moment things get Darker and Edgier.
G.I. Joe: Cobra #12 in which Chuckles shoots Cobra Commander dead and then blows the body, the base and himself up with a nuke.
The X-Men storyline "Fatal Attractions" had Wham Moment after Wham Moment. Including:
X-Force #25 - Cable returns, only for new foe Exodus to beat him senseless!
Uncanny X-Men #304 - Magneto returns and a disillusioned Colossus joins the Acolytes!
X-Men vol. 2 #25 - Magneto rips out Wolverine's adamantium and Xavier mindwipes Magneto!
Wolverine vol. 2 #75 - Wolverine discovers that there's more to his claws than the adamantium!
Cerebus has several. Most notably in issue 76 when Weisshaupt tells Cerebus that there are two other Aardvarks.
Journey into Mystery 645 suggests a very different view of all of kid!Loki's adventures up to that point
The 1969 Justice League comics "And So My World Ends..." was this for the Martian Manhunter, killing off his planet, his entire species, driving him out of the Justice League, and setting the tone for ever story featuring J'onn J'onnz to be published after the fact.
Habibi by Craig Thompson features a monster of a middle chapter (approximately 130 pages long) depicting Habibi's adolescence after being separated from Dodola. An epic and turbulent Coming-of-Age Story in it's own right, the wham moments are his choosing to be castrated out of guilt for lusting over his caregiver and the conclusion in which he finds Dodola in the sultan's palace at long last, and stages their escape, at once uniting the Two Lines, No Waiting and resolving what was up till now the main conflict in a single Crowning Moment of Awesome.
In the Dark Horse comics, it's revealed that the lead commander is really a Simeon.
IDW comics has issue 6 of the Duane Swierczynski/Simon Gane comic which has Claire getting a vision from Mothra of the Space Monsters coming to Earth. In issue 8, we finally see their arrival.
In Rulers of Earth, we are shown SEVERAL Wham Episodes. In issue 1, we are given the implied revelation that the Cryog sent the Space Monsters. Issue 6 introduced Jet Jaguar, issue 8 has the Devonians betray the Cryog, in issue 9, it's revealed that this is not the first time the Kaiju have appeared, nor is it the first time humanity interacted with them and issue 12 the Cryog Emperor was revealed to have survived.
ElfQuest has had its share over the years. Notably, the elf-troll battle in the original series which lead to the death of one character and the serious wounding of another; and the moment in Kings of the Broken Wheel when Rayek takes Skywise, Leetah and the twins into the future, leaving the rest of the Wolfriders stranded in the present.
The Apocalypse War gave us the large-scale destruction of much of Mega-City One, the total destruction of its Soviet rival East-Meg One at Dredd's hands and a new Chief Judge, as well as setting the stage for three decades of revenge.
Origins: Much of the backstory of the Dreddverse is revealed, and Fargo, the founder of the Judges delivers this Wham Line:
Forever Evil: The Justice Leagues 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.
The final issue is also quite whammy, with Luthor figuring out Batman's secret identity. Oh, and The Anti-Monitor is back and killing universes in preparation to fight Darkseid..
Young Avengers vol.2 has three in a row. First issue #8 ending on Prodigy kissing Hulkling. In next isssue Billy and Teddy break up, Teddy quits the team, and it turns out his therapist is Leah of Hel. Final series of whams comes in issue #10 - Loki was manipulating both Young Avengers and Mother from the very begining, Leah joins forces with Mother and gives her Hulkling. Then comes issue #12 in which turns out Leah isn't real Leah, but Loki's self-loathing given female form.
X-Men: Legacy vol.2 has one in issue #5 - turns out Up until this point David was manipulated by Ruth's evil brother, allowing him to try kill his sister. Then in issue #6 it seems like David has things under control and future looks bright for him, until he is defeated in his own mind by Evil Charles Xavier, who reveals in future David will destroy all mutants and Ruth is the one who will save them - by killing him. Then comes issue #18, in which Emma Frost makes David fall into despair, allowing Evil Xavier to escape and steal Luca's body. Then comes issue #22 where David loses control and turns into monstrous entity that starts absorbing minds of every mutant on the planet.
Simon purrier's run on X-Force seems to followsuit. Issue #5 reveals Marrow was pregnant and undergoing process that gave her back her powers made her misscarriage. And only pages later it turns out she undergo this process willingly, knowing the risk very well. And in issue #6 it turns out Cable is slowly dying and prolonging his life with cloned bodies, while Meme has been dead all along - Meme characters interact with is really comatose Hope, secretly using her powers to be with her father.
The Avengers tend to have fewers wham episodes than most, but this makes them all the more dramatic when they do happen:
Issue 16. Up until Stan Lee wrote this issue, the comic had been about the adventures of some of Marvel's biggest star characters, but in this issue, all the founding Avengers, including Iron Man and Thor, leave the team indefinitely, leaving Captain America in charge of a team of new recruits. Even more shockingly, all the new recruits - Hawkeye, the Scarlet Witch, and Quicksilver - had originally been introduced as villains. By the end of the issue, all the original stars are gone and the book's focus has completely changed. It's an example of how a comic can do a Nothing Is the Same Anymore episode with no deaths.
The classic "Under Siege" story arc from volume 1, issues 275-277. It starts out as a normal day at the Avengers Mansion, only for the Masters of Evil to suddenly attack and promptly curb-stomp the Avengers. They than toss the team out of the Mansion and take it as their new base. It concludes in an epic battle between the Avengers and Masters that concludes with Blackout dead, the Super-Adaptoid loose again, Jarvis in the hospital after being brutalized by Baron Zemo, and the Mansion wrecked. It really shook things up for the team, making them feel more vulnerable than ever both in and out of universe. The events of the arc haunted the Avengers for some time afterwards, with Captain America and Hercules spending the next several issues in a deep depression because of their perceived failures.
"Ultron Unlimited", in which Ultron officially crossed the Moral Event Horizon by wiping out a country. Yeah he had been evil before than, but that made it explicitly clear that the bastard is completely beyond redemption.
Original Sin was basically a massive Wham Episode for the whole Marvel Universe; Nick Fury murders the Watcher and takes his place as punishment, Dr. Midas and the Orb return, Ulana is going to give birth to Uatu's son soon, Daredevil learns the truth about his mother, Captain America learns about the mindwipe the Illuminati performed on him, it's revealed that Iron Man was indirectly responsible for Bruce Banner becoming the Hulk, Thor is rendered unworthy of his hammer, the 1970's Avengers are gathering again for unknown reasons, and Dum Dum Dungan has been Dead All Along, having been replaced with an LMD after the real one died in the Sixties. Yeah.
Issue 6: Fortress Maximus goes insane, Rung is accidentally shot by Swerve (which puts him out of commission for several issues), Whirl reveals why he was kicked out of the Wreckers, Red Alert is going off the deep end, and the being in the basement is Overlord.
Issue 14: Drift and Brainstorm are working for Prowl and are responsible for Overlord being on the ship. Overlord escapes his containment.
Issues 15-16: Overlord kills Pipes and Rewind and fatally wounds Ultra Magnus. Drift is exiled and Tailgate has three days to live.
Issues 17-21: The crew finds Luna-1 which is filled with sparks and all of the villains the crew has encountered so far, all lead by the now-insane Chief Justice Tyrest. They discover that Ultra Magnus is actually Minimus Ambus, Ambulon and Dai Atlas die on Luna 1, and the Lost Light gets attacked by Legislators who behead Deftwing.
Issues 32-33: The crew arrives in Sector 113 and find an alternate Lost Light where the DJD slaughtered everyone but Rewind. While there they discover that Brainstorm is a Decepticon agent and he proceeds to use his briefcase to knock out the crew and make his getaway.
It's practically tradition for any run on Daredevil to have at least one Wham Episode. Some examples:
The "Born Again" arc in Frank Miller's run: Kingpin learns Daredevil's secret identity and uses this to basically ruin his life, culminating in Matt being forced to break his "no killing" code to save New York and than utterly devastating Kingpin's forces.
The conclusion of Brian Michael Bendis's run: Daredevil manages to screw Kingpin over, but only by becoming the new Kingpin. He than gets unmasked to the public and thrown in jail.
The last stretch of Ed Brubaker's run: Mister Fear drives Milla Donovan into a paranoid breakdown just to screw with Daredevil.