Cassandra Nova is a powerful and malevolent super-villain from Marvel comics who is primarily a recurring antagonist towards the X-Men, being seen as a parasitic "evil twin" of Charles Xavier that would grow to become an especially personal enemy of the group, being the exact opposite of her "brother" and thus a murderous monster both in personality and capabilities.
History[]
The entity which would become Cassandra Nova was part of what the Shiar called the Mummudrai, a type of astral parasite that was believed to represent the evils of a developing infant, a dark "twin" that every living being faced within the womb.
Cassandra Nova was more powerful than the usual Mummudrai, so much so that Charles Xavier fought her within the womb and seemingly vanquished her only for the parasite to somehow survive and evade notice for many years, during which she achieved what most of her kind could not and made herself a physical body by which to exist fully separate from her "brother".
Yet due to her parasitic nature she would forever hold a dark obsession with Xavier, seeking to destroy everything he held dear and developing a twisted madness in which she viewed all life save for Xavier and herself as false - this led her to commit many unspeakable crimes, among the worst being the massacre of Genosha, which resulted in the deaths of over sixteen million mutants.