Onslaught was born when a fed-up Professor X ran over Magneto's mind like a steamroller running over a wasp. A combination of Xavier's and Magneto's dark thoughts, the ones Xavier fought so hard to repress, Onslaught ultimately became a mass of reality-warping, almost unstoppable psychic energy, eventually shifting his goals from "kill all humans to save the mutants" to "just kill everybody." Stopping him required the ultimate sacrifice for many heroes-- Captain America, Thor, Iron Man and the Fantastic Four ended up on another plane of existence where some of them had to be drawn by Rob Liefeld. Onslaught returned in a sense recently, as the Red Skull assumed the abilities of the departed Professor X in the grossest way possible.