Broken Frontier

Following the restoration of the Marvel Universe proper at the conclusion of Age Of Apocalypse in early 1996, the X-Men did not have long to wait before they were catapulted into another reality-bending crossover. Moreover, the threat they confronted stemmed once again from the past actions of their erstwhile mentor, Charles Xavier, who had inadvertently created the entity that give his name to the Onslaught Saga.

Onslaught's arrival was foreshadowed for months prior to the beginning of the crossover proper, but his origins truly lay even further back, in 1993's Fatal Attractions. At the close of that storyline, Xavier had telepathically lobotomized his nemesis Magneto as retaliation for a near-fatal attack on Wolverine. While Magneto was instantly incapacitated, Xavier's own mind acquired some of the anger that had festered within Magneto. Joining with Xavier's own repressed rage towards the intolerance of humanity, this evil influence grew in power until it gained consciousness as a being of psionic energy – Onslaught.

Onslaught's power grew rapidly, and he started to manifest to various characters in the X-verse, manipulating events to his own ends, probing the weaknesses of Xavier's students and learning how to take on physical form by merging with Xavier. He appeared in the flesh in X-Men #53, massive in size and with a costume similar to Magneto's. Onslaught engaged the X-Men in battle, laying waste to the school. 

The team was saved only by the timely intervention of Bishop, who realized that a video recording of Jean Grey which had guided him into the past and seemed to implicate Gambit as a traitor to the X-Men was in fact referring to Xavier and his creation of Onslaught. By defending the X-Men at this juncture, Bishop seemed to avert the creation of his own future timeline.

Nevertheless, Onslaught continued his reign of terror, attacking New York City with Sentinels in a bid to begin a global apocalypse. The non-mutant Marvel heroes became involved at this point, as well, embroiling the entirety of the publisher's slate in the crossover. The X-Men and the Avengers traveled to New York to protect Franklin Richards, powerful mutant son of Mister Fantastic and the Invisible Woman, from Onslaught, while X-Force remained in Westchester with the equally powerful psion Nate Grey.

Unfortunately, both groups' efforts were for naught. Onslaught kidnapped both Franklin and Nate, tapping into their abilities to boost his own telepathy and electromagnetic manipulation to almost infinite levels. In a final confrontation in Central Park, Onslaught manages to resist the combined assault of the X-Men, the Avengers, the Fantastic Four, the Hulk, Namor and Doctor Doom. His physical form is destroyed by their attacks, but this only increases his power, and he begins to create a second sun with which to destroy the Earth. 

Realizing that mutant energies increase his power, the heroes theorize that Onslaught's psychic energy might be contained by non-mutant human bodies. With that, the Avengers, the Fantastic Four, Namor, the Hulk and Doom cast themselves into Onslaught's form while the mutants redoubled their assault.

The attack succeeded, after a fashion, although apparently at the cost of the lives of those who entered Onslaught. In reality, Franklin Richards fashioned a pocket universe in which the heroes survived and the age of superpowered beings was just beginning, an experiment known as Heroes Reborn that would last for a year of real-world publishing time. The X-Men were left to deal with the repercussions of having seemingly slain the world's greatest defenders, as well as the reality of the darkness that lurked inside their leader.

The Onslaught Saga was not the X-over's finest hour. While it did change up the formula by bringing in other Marvel characters and led logically out of previous events, the storyline itself was muddled and unwieldy. The complete arc ran through more than 50 issues of nineteen different series, as well as several one-shot specials*. 

Onslaught himself, meanwhile, seemed to be more of a deus ex machina than a well-rounded character.  His motivations and goals beyond petulant rage and a desire for destruction were never fully established, and required a one-shot of their own ( X-Men: Road To Onslaught) rather than being explained in the course of the main plot. This might be due to the lack of a main plot, as dozens of characters moved through both their own books and others' without clearly established goals, narrative through-lines or subplots. 

The Onslaught Saga was also clearly the beginning of the vilification of Professor X, a trend which has continued forward and is a key plot point of ongoing storylines in the X-verse. While Xavier's actions and motivations had been questioned before, it was only with this arc that he became a threat, someone for the X-Men to actively mistrust. Once the guiding light of enlightened mutantkind, Professor X has become a figurehead to be trotted out when Cyclops needs a moral punching bag to work over, an ignominioius fate for a beloved character. 

Perhaps the only bright spot in this gloomy crossover was the fanboy Easter egg of the explanation of the "traitor" video, and even that has been rendered somewhat moot. While it was implied that Bishop's timeline had been averted, it has repeatedly been shown to still exist, most recently during Messiah CompleX. This can, of course, be explained away by hand-waving over comic book physics and alternate universes, but should be noted nonetheless. 

Other consequences of the Onslaught Saga have vanished; the Marvel heroes have long since returned from the Reborn universe, and Onslaught himself did not appear again until the overlooked and quickly-forgotten Onslaught Reborn event following House Of M . The Onslaught Saga was an overblown and ill-conceived event that seemed to exist solely to give creators an excuse for a universe-wide team-up, an opportunity that was squandered because of the unmanageably large cast and poorly defined villain. 

*For completeness' sake, the issues in question are: Amazing Spider-Man #415, Avengers #s 400-402, Cable #s 32-36, Excalibur #s 99-100, Fantastic Four #s 414-416, Generation X #s 18-19, Green Goblin #12, Incredible Hulk #s 444-447, Iron Man #332, Punisher #11, Spider-Man #72, Thor #502, Uncanny X-Men #s 333-337, Wolverine #s 104-105, X-Factor #s 124-126, X-Force #s 57-58, X-Man #s 15-19, X-Men #s 53-57, the one-shots Onslaught: X-Men, Onslaught: Marvel Universe, Onslaught: Epilogue, X-Men: Road To Onslaught, and various appearances of Onslaught in months prior.  Given the hazy chronology of the event, any suggested read order would be at best nominal.