Onslaught

Onslaught is the name of an extremely powerful, malevolent mutant thought-form created by the combined consciousness of Professor Xavier and Magneto - at first Onslaught wanted to follow the mindset of Magneto and have mutants rule the world, yet after he saw a vision of the Age Of Apocalypse time-line from the mind of Dark Beast he became convinced that neither human nor mutant could rule the world and set upon exterminating both races (in effect wiping out pretty much all life on Earth).

To achieve this end he kidnapped Frankling Richards and Nate Grey in order to add their formidable psychic abilities to his own, making himself all but a god -this new, immensely powerful Onslaught would fight with the Avengers, X-Men, Hulk and Fantastic Four.

After much fighting Hulk was able to break Onslaught's armor - destroying the monster's physical form but exposing his raw psychic energy in the process, in order to defeat him many of the non-mutant heroes sacrificed their lives by entering Onsalught's exposed form (the mutant heroes could not do so as he would of fed of them and become even more powerful) - weakening him enough for the X-Men to finish him off.

This battle led to the Heroes Reborn storyarc and made Onslaught one of Marvel's "love or hate" villains, with some fans liking him and others seeing him as little more than an elaborate tool for needless retconning of the Marvel universe.

Regardless of this Onslaught was, for better or worse, one of Marvel's most powerful villains and his actions would have profound changes on the universe as a whole..