'''Onslaught''' was a 1996 MarvelComics CrisisCrossover involving nearly every superhero comic book they published at the time. The ComicBook/{{X-Men}}, [[Comicbook/TheAvengers Avengers]], the ComicBook/FantasticFour and other heroes fight an incredibly powerful OmnicidalManiac which results in the "[[NotQuiteDead deaths]]" of every hero involved (except the mutants, {{Spider-Man}} and a Banner-less [[ComicBook/IncredibleHulk Hulk]]).

The X-Men suffer several attacks by minions of the seemingly omnipotent entity known only as "Onslaught", who appears to know them well. He is also the one behind the recent stealing of the [[MechaMooks Sentinel]] fleet from a government base. When Gambit and Jean Grey go undercover to learn more about an unrelated GovernmentConspiracy, they are exposed but get an unexpected help from Onslaught. Later he tries to bring Jean over his side by showing her [[HotForStudent the dark long-repressed thoughts of her mentor Charles Xavier]]. She refuses. {{Juggernaut}}, who was [[TheWorfEffect attacked by Onslaught as well]], comes to the X-Mansion and asks Jean to look for its true identity [[MindReading inside his head]]. She does and finds out that Onslaught is none other than Professor X.

It turns out that when Xavier brainwashed {{Magneto}} several months before (to cause him to stop being a threat after he cruelly stripped the adamantium off {{Wolverine}}'s bones), a part of the latter's psyche merged with his own mind. Under the stress of constant failures and new misfortunes the Professor became more and more frustrated, until he developed a split personality, which eventually [[SplitPersonalityTakeover took a life of its own]] and was able to manifest in the corporeal world. Onslaught nearly kills the X-Men before leaving to capture [[ComicBook/FantasticFour Franklin Richards]] and ComicBook/XMan in order to suck out their powers and become [[PhysicalGod a living god]]. That brings him into conflict with both the Avengers, who were approached by X-Man, and Franklin's family, the Fantastic Four. Despite that Onslaught successfully kidnaps both of his targets, encircles Manhattan with Sentinels and prepares to TakeOverTheWorld.

The heroes (and DoctorDoom) eventually manage to defeat him but in order to do that most of them have to enter his non-corporeal form, which the X-Men then have to destroy, making it look like they killed them. Professor X is imprisoned for Onslaught's crimes and the anti-mutant sentiments become stronger than ever.

(Unknown to anyone, the heroes survived as they were recreated -by Franklin's powers- in another world, with no memory of their previous lives, as seen in the sequel "HeroesReborn".)

After the events of ComicBook/HouseOfM Onslaught came BackFromTheDead in the aptly named ''Onslaught Reborn'' miniseries.

!!Tropes used in Onslaught:

* ADarkerMe: What happens when the dark sides of Xavier and Magneto combine.
* AGodAmI: Onslaught at one point actually calls himself a god.
* AllThereInTheManual: ''X-Men: The Road to Onslaught'', a special comic that told [[ExactlyWhatItSaysOnTheTin the backstory of Onslaught]] in chronological order.
* AWizardDidIt: Though other mutant heroes couldn't enter Onslaught's field (they'd get possessed) the Avenger's Scarlet Witch was able to make an exception for herself with her powers.
* BittersweetEnding: Onslaught is destroyed, but the Fantastic Four and many Avengers are seemingly killed in the process. The public also thinks the X-Men are responsible, triggering severe anti-mutant hysteria and Xavier being taken into government custody. Franklin Richards is orphaned and taken in by Generation X.
* BroughtDownToNormal: [[spoiler: Charles Xavier]] who lost his powers afterwards.
* CapcomVsWhatever: Onslaught served as the final boss of the first Marvel Vs. Capcom game, where he was the resident SNKBoss.
* TheCavalry: "And yet... through the haze of dust, Xavier sensed a coming... of hope, of courage, of marvels."
* CrazyPrepared: Turns out Professor X has made specific plans and instructions on how to deal with any X-Man (including himself), should they ever [[FaceHeelTurn go rogue]].
* DangerouslyGenreSavvy: Onslaught knows everything Xavier knows, meaning he can anticipate how many of the various heroes will act.
* DarkestHour: Onslaught announces his intention to wipe out ''all'' of humanity and then crushes the X-Men in battle. Fortunately, the Avengers and Fantastic Four arrive during the lull in fighting.
* DidYouJustPunchOutCthulhu: The Hulk literally punched Onslaught so hard that he destroyed his physical form.
** Onslaught punched him back hard enough to knock Bruce Banner out of the Hulk.
* EnemyMine: Doom sides with the heroes and helps them against Onslaught. When he tries to collect an energy sample for his own ends, though, Hawkeye and Iron Man take him down.
* EnemyWithin[=/=]EnemyWithout: Xavier's evil side had already manifested in physical form once before in the ''X-Men vs Franchise/{{Micronauts}}'' series.
* EvilTowerOfOminousness: Onslaught created one in Central Park.
* ForgotICouldFly: Magneto has limited telepathic powers of his own, it was likely because of this that the mindwipe backfired. He just rarely uses them these days.
* GenreSavvy: None of the heroes are dumb enough to blindly trust Doom, but the more pressing threat keeps them distracted. Iron Man and Hawkeye, however, stay focused and are ready Doom when tries something.
* HeroicSacrifice: The heroes. (Doom was dragged along involuntarily by Iron Man.)
* HoistByTheirOwnPetard: The Sentinels were designed to protect humanity from mutants. Onslaught enjoyed the irony of reversing that intent.
* HumansAreTheRealMonsters: The original motivation of Onslaught was to KillAllHumans for their oppressing of mutants. Then he saw the world [[ComicBook/AgeOfApocalypse X-Man came from]] and decided that [[NotSoDifferent mutants are bastards]] [[KillEmAll too]].
* LiteralSplitPersonality: Onslaught and Xavier partway into the story, as well as Hulk and Banner at the end.
* MechaMooks: Sentinels.
* NiceJobBreakingItHero: Separating Xavier from Onslaught. Instead of destroying him, it freed Onslaught from any sense of Xavier's morality and influence.
* NotSoImaginaryFriend: Franklin's "invisible friend" Charlie. Onslaught did this to ensure he could kidnap Franklin right out from under the FF.
* OffWithHisHead: One memorable scene had the Ben Reilly Spider-Man defeat a Sentinel by (with great effort) ripping its head off.
* OmnicidalManiac: See above.
* RedSkiesCrossover: ''Wolverine'' # 103 was labeled as tie-in but had no connection whatsover.
** The titles displaying the Crossover's logo were generally separated into two types, with directly affected titles branded "Onslaught: Phase (1, 2, or 3, depending on the month)" while the more Red Sky types were billed as "Onslaught: Impact (1, 2, 3)". Most of the "Impact" issues featured the heroes (e.g. SpiderMan) facing Onslaught-controlled Sentinels. Interestingly enough, there was an issue of ''Comicbook/ThePunisher'' that had criminals hiding out inside the remains of a destroyed Sentinel.
* SoProudOfYou: According to the narration during the beginning of the final battle, Xavier "never was prouder of his students."
* SplitPersonalityTakeover
* SuperpoweredEvilSide: While Xavier is one of the world's most powerful telepaths, Onslaught also had [[SuperWeight telekinesis and magnetism]]. He grew even more powerful upon capturing Franklin Richards and Nate Grey.
* TrailersAlwaysSpoil: The Heroes Reborn line was advertised on the covers of Onslaught issues.
* UntouchableUntilTagged: Onslaught was unstoppable until cracked his helmet.
* WhamEpisode: Many of Marvel's major heroes are thought dead (actually sent to a pocket world), all remaining heroes have to pick up the slack (opening the door for the {{Thunderbolts}} development), the X-Men are blamed for the whole mess, and Hulk is split from Banner.
* WhamShot: The image of Charles Xavier being inside the Onslaught armor after ripping the Gem out of Juggernaut's armor.
* WhatHappenedToTheMouse: In an issue of Spider-Man, Peter Parker and [[ScarletSpider Ben Reilly]] defeat a few Sentinels and run off at the end of the issue to help the other heroes fight Onslaught. They never show up in any of the battles with no explanation.
* TheWorfEffect: Early on Juggernaut got beaten up by Onslaught to establish him as a big threat. [[WritingByTheSeatOfYourPants It was before writers made up their minds about just anything else of him.]]
** It becomes unintentional FridgeBrilliance after TheReveal of Onslaught's identity: If you become evil and all-powerful, one of your first targets would be the invincible-til-now man who has bullied and tried to kill you from childhood to the present day.