In a major battle with one of the X-Men's most powerful enemies, Magneto, Xavier temporarily mind wiped Magneto out of anger. Magneto nearly killed Wolverine because he brutally ripped the adamantium off of Wolverine's bones and through the pores of his skin. Ouch. Because of the guilt he felt, Magneto had been Xavier's best friend once, Charles developed a split personality. Xavier also absorbed some of Magneto's personality and powers. This personality, known as Onslaught, threatened the entire Marvel Universe, until the combined efforts of the Fantastic Four, Avengers, and X-Men stopped him. I have found 3 versions of Onslaught for mugen.
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By Excursion & Stever76 (Infinity Mugen Team)
I don't really like the Overlander version all that well. It's slow and it does not have many moves. The OnslaughtX version is a lot of fun, but a tad overpowered. It's mega blast move is usually enough to stop anyone. Two of them, and the fight is usually over. Kinda cheap, but fun to do. I only wish that he could kick and punch. As for the symbiote version. It's the fastest, easier to control and has a lot of moves. I'm not sure if any of these are complete but I like the Symbiote Onslaught and Onslaughtx's Onslaught.
Here are the video tests I ran.