Wolverine Vol.1 #104Onslaught was created when Professor X mind blasted Magneto after Magneto pulled the adamantium from Wolverine's body. Professor X's dark side manifested inside him until it became Onslaught, a being of great power. It took the Avengers, Fantastic Four, X-Men and many others to elimate Onslaught, many super heroes died in the battle.