A Grant Morrison creation, Cassandra Nova's origin story is a bizarre one, but despite being the identical twin of Charles Xavier who he strangled to death in the womb, this psychic entity ended up causing all sorts of problems for the X-Men. In fact, she wiped out millions of mutants when she unleashed a gigantic Sentinel on Genosha and switched minds with her brother, later killing him while he was trapped in her body. One of the most evil villains the X-Men have faced, Cassandra even had a huge impact after being killed off herself. She implanted hypnotic suggestion in Emma Frost's mind which forced her to turn on her teammates, and while Cassandra was once again defeated, it remains unknown whether her consciousness has been lost forever or if she may be hiding out in the mind of one of the X-Men. Now that's a pretty scary thought!
