20 Most Powerful X-Men Villains Of All Time
6. Cassandra Nova
Whilst Professor X has often been a major threat to his own X-Men himself, even worse is the Actually Evil Xavier. Cassandra Nova is the unborn twin of Charles, who he killed during a psychic battle in the womb when he realised she was pure evil. Nova's the opposite number to Xavier, with all the same abilities and none of the morals.
In fact, she might even best Xavier at times, having at one point switched brains with Charles just in time to shoot her own body and put it in a coma. Her telekinetic abilities are almost as incredible as her telepathic ones, with her first assault on the X-Men involving completely incapacitating Cyclops and rending the flesh off of Wolverine's arm.
Cassandra Nova has thankfully (for the X-Men) mostly disappeared from the comics, but during her brief appearances she set herself up immediately as one of the most powerful enemies they had ever encountered. She incapacitated most of them without lifting a finger, and built a thousand new Sentinels that tore a whole mutant nation apart.