A member of a species of bodiless parasites composed of living emotional energy known as the Mummudrai, legendary for appearing to every being in-utero shortly before birth as their ultimate opposite, the creature that would become known as Cassandra Nova appeared to the unborn Charles Xavier, copying his DNA to create its own body, in effect becoming his twin sister. Sensing its monstrous thoughts, Charles attempted to kill the creature resulting in its physical body being stillborn; however, the mummudrai survived as chaotic cellular matter and clung to a sewer wall for decades, rebuilding its physical form and perfecting its effort to mimic human traits. Perceiving the womb it became aware was the entirety of the universe, Cassandra believed that only she and Charles were real, and that killing him meant mastery of the universe as she saw it. To that end, Cassandra lured dentist Donald Trask, nephew of Sentinel creator Bolivar, and with his unwitting help she took control of a group of “wild” Sentinels, designed to adapt to their environment. Cassandra then sent the Sentinels to slaughter the population of the island nation of Genosha in the hope of weakening Charles, and subsequently confronted Charles and his mutant students, the X-Men. She was seemingly defeated after the X-Man Emma Frost broke her neck. Unbeknownst to the X-Men, Cassandra had actually switched minds with her brother. Trapped in Cassandra’s broken body, Charles attempted to warn the X-Men; however, Cassandra – in Charles’ body – used a gun to end the faux threat while at the same time infecting the X-Men with bacterial Nano-Sentinels she had secreted in her bloodstream. These microscopic machines began systematically dismantling the X-Men’s white cell immune structures, slowly weakening them.