20 Most Powerful X-Men Villains Of All Time
4. Onslaught
Two of the most powerful mutants in X-Men history are Magneto and Charles Xavier, so it stands to reason that putting the worst aspects of both of them would make for a particularly nasty villain. Onslaught was the result of Professor X wiping out the bad side of Magneto's brain during a fight, with the side effect of him absorbing those parts.
They remained dormant for a while before Xavier had a psychic break and became Onslaught, keeping the good parts of his personality locked up whilst a psychopathic combination of his and Magneto's worst qualities became a huge, super-powerful giant clad in spikey Magneto-esque armour.
The lack of sanity apparently let his psychic powers run wild, too, to the point that the final confrontation between Onslaught and the combined might of the Marvel universe's heroes resulted in the death of the Fantastic Four and most of the Avengers. Charles Xavier is enough trouble on his own; his powers unchecked are almost impossible to beat.