Cassandra Nova

Cassandra Nova is a very powerful enemy of the X-Men and also one of their more recent ones, though her origins go back as far as the birth of Professor Xavier himself - a member of a parasitic race of psychic beings Cassandra attached herself to the unborn Charles Xavier while still in the womb, in essence becoming his twin sister but also embodying the darkest parts of Xavier's mind.

Becoming aware of her presence - and the threat she posed to the world - the infant Xavier seemingly vanquished Cassandra before even being born but somehow she would survive and grow mad, becoming convinced that she and Xavier were the only real things and that everything else was a lie.

Cassandra was also deeply weakened from the battle with the infant Xavier and it took her years to reform to a level of power suitable to become a threat to the X-Men, her most infamous act of villainy was when she commanded an army of Sentinels to mercilessly massacre over 16 millions mutants on the island of Genosha - making her one of the most destructive villains of the entire series (at least in the mainstream reality).

Cassandra Nova is not content with this massive lose of life though and is still planning more mayhem, being described as Xavier's dark half personified, seeking destruction and genocide so that she may accomplish what she sees as her true goal in life: to erase every "illusion" Xavier holds dear.