According to Shi'ar legend, everyone of us is born with a mummudrai, our emotional antithesis in the astral plane. It acts like a bodiless parasite inside the womb, but it usually never comes to term. 

Professor Charles Xavier got an exemption to the rule. With his genetic mutation, Charles inadvertently enabled his mummudrai to build a body for itself in the womb. Put simply, an evil twin was growing alongside a fetal Charles, and he knew it even then. A battle ensued in the uterus. (Was it set to Massive Attack's "Teardrop"?) Charles was born and the mummudrai perished. 

Here's the icky part. Unbeknownst to Charles, the stillborn mummudrai survived in the sewer, where it slowly grew from a clump of cells to become Cassandra Nova.

As Professor X's twin sister, Cassandra is more than a match for one of the planet's greatest telepaths. Apart from having Charles' psionic powers, Cassandra can devour minds and perform astral projection like any mummudrai.

Cassandra instigated the genocide of 16 million mutants in Genosha and created the HIV-like Nanosentinels. Switching bodies with Professor X, she was able to manipulate his wife Lilandra into ordering the slaughter of the X-Men by the Shi'ar Imperial Guard. 

Jean Grey and Emma Frost outwitted Cassandra by letting her use a Cerebra where Charles' consciousness lay in wait and having a shape-shifting extraterrestrial pose as her original body for her to jump back into.