The final boss of Marvel Vs. Capcom, Onslaught, is a villain who was from a major Marvel storyline that ran in 1996. This storyline ran through many different Marvel comics and to get the full story you had to buy X-Men, X-Men Unlimited, X-Force, X-Factor, X-Man, Cable, Incredible Hulk, Fantastic Four, Iron Man, The Avengers and Spider-Man comics -and what I've just listed there isn't everything! For a complete guide to what parts of the Onslaught storyline appear in the comics, please check this page from UncannyXmen.net.
Instead of hunting down individual comic books, your best bet if you want to read the Onslaught saga is to get these collected editions. Prelude to Onslaught deals with the events that will later cause Onslaught to appear, whilst Onslaught: The Complete Epic is a 4 volume series which collects everything to do with the character, what he does, the effects his actions have on other super heroes and how the X-Men and other Marvel heroes try to stop him.
The start of it all: X-Men Vol 2 #25
Released: October 1993
Magneto rips out Wolverine's adamantium skeleton -unaware that his actions send Xavier over the edge!
This comic was part of the Fatal Attractions storyline. If you haven't read it, it involves Magneto and his Acolytes taking over Avalon, a space station which used to be Cable's base of operations. At Avalon, Magneto and his crew are safe to use an electromagnetic pulse blast on the Earth which causes major damage and other problems to anything electrical.
Professor Charles Xavier gets together some of the X-Men and heads out to stop Magneto from doing any more damage. He is able to walk again by using a special exoskeleton and he, Jean Gray, Gambit, Quicksilver, Rogue and Wolverine head to Avalon. There they quickly manage to take over the space station but Magneto soon finds them and a massive battle starts.
Magneto is about to kill his own son (Quicksilver) when Wolverine manages to stop him at the last second. However Magneto becomes enraged and turns his attention to killing him instead. Using his magnetism powers, he rips Wolverine's adamantium skeleton completely out of him. Wolverine is seriously injured but alive, thanks only to his mutant healing ability.
But what Magneto has done to Wolverine proves to be too much for the usually calm and non-violent Xavier to cope with. Having seem him hurt people one time too many, Xavier uses his telepathic powers to completely shut down Magneto's mind, leaving him in a near dead state. Bishop arrives to pick the team up and they head back to Earth. Wolverine eventually recovers, discovering that his claws were actually bone, rather than just adamantium.
Shutting down Magneto's mind had a dire consequence -the birth of Onslaught!
What Xavier didn't realise though was that during the psionic contact he had with Magneto as he shut down his mind, Magneto's anger, his grief and his lust for vengeance made it's way into Xavier's own consciousness. For a few years, nothing came of this. However several events would happen that would make Xavier lose faith in his dream of peace between mutants and humans. A virus known as the Legacy Virus killed several mutants. Xavier failed to rehabilitate a violent mutant called Sabretooth. Anti-mutant protesters were also on the increase, resulting in the death of a young mutant near the Xavier School of Higher Learning.
This would be the point where Onslaught appeared for the first time and finally showed himself to others. One of his first actions would be to crush Juggernaut, as seen on the front cover of Uncanny X-Men Vol 1 #332. However things would only get worse from there. Onslaught takes over Charles Xavier completely and attempts to turn the X-Men into soldiers who will go out and fight humans, who Onslaught sees as evil. They refuse and this is the point where the story really begins. The X-Men and many other Marvel heroes band together to stop Onslaught from taking over the world.
The video game: Marvel Vs. Capcom
As stated earlier, Onslaught is the final boss in Marvel Vs. Capcom. Much of his dialogue is straight from the comics. Click on an image to hear the relevant voice clip:
This line was made shorter and used as a win quote for Onslaught: