The Mummudrai
According to Shi’ar legend, all sentient beings face their own mummudrai - their ultimate opposite - in the womb, shortly before birth. It is one’s first experience of the alien, the “other.” The different. Whether legend or not, Charles Francis Xavier faced such an entity shortly before birth.
In truth part of a bodiless, parasitic species, born without true flesh or real substance, the mummudrai copied Charles Xavier’s cells to build its body, in turn copying his latent mutant abilities - in effect becoming his twin sister.
Though still gestating, Charles’ mind detected Cassandra and her monstrous thoughts. He sensed a raging hunger, vast enough to consume him and the entire world. So, Xavier’s first conscious act was to lash out, in their mother’s womb, and to kill his twin. Cassandra, however, was not defenseless and retaliated with a portion of what could become the most powerful telepathic mind on the planet. The attack, however, affected their mother, who was climbing the steps of their home, and she fell. When their mother eventually gave birth, Charles was healthy, but Cassandra was stillborn. [New X-Men (1st series) #121, 126]
Despite her death, Cassandra endured. Born without a body, she survived as chaotic matter - living emotional energy, formless and unique. For forty years, she clung to a sewer wall, waiting until she was strong enough to walk free. During that time, while she formed a new body, basing it off of Xavier’s genetic structure, Cassandra grew convinced that the universe in which she existed and the womb in which she first gained consciousness were one and the same. In all the universe, only she and Charles were real. This would no doubt ease the horrific actions she would later take.
The questions as to how Cassandra survived during those forty years, how she came to complete the construction of her new body - based on Charles Xavier’s - and even how she came to claim the name of Cassandra Nova are unclear. However, by the time she made her presence known to the world, Cassandra Nova seemed for all the world to be an educated, aged woman, very well schooled in the powers of persuasion.
The first known act of Cassandra Nova was the recruitment of Donald Trask III, nephew and last known relative of Sentinel creator Bolivar Trask. Finding him in Albuquerque, running a dental practice, Cassandra introduced herself as the world’s foremost evolutionary biologist and explained her findings: an extinction gene would cause the 
extinction of human race within four generations. These claims, it should be noted, were also discovered by X-Men geneticist Hank McCoy, the Beast. [New X-Men (1st series) #114]
Explaining humanity’s predicament, Cassandra took Trask to an abandoned Sentinel Master Mold factory in the hills of Ecuador. There, Cassandra demonstrated his purpose: the Wild Sentinels, which the Master Mold manufactured, would only respond to instructions from a Trask. Under the direction of Cassandra, Donald Trask ordered the construction of two immense Mega-Sentinels, which were immediately dispatched to the island nation of Genosha, off the coast of Africa.
Trask’s usefulness, however, was soon at an end. With her ability to mimic genetic structure - much as she had done Xavier’s over the past few decades - Cassandra had spent the previous ten hours reading and copying the three billion base letters in Trask’s DNA sequence. Without a second thought, she eliminated him, then turning her attention to the next stage of her plan.
Arriving, almost as if on cue, were Cyclops and Wolverine. At the Xavier Institute, Xavier had become aware of Cassandra’s presence through the recently completed Cerebra, and had sent the two X-Men to investigate. Arriving at the Master Mold factory, the two faced the facility’s Wild Sentinels and were easily captured. Waiting for her pawns to recover, Cassandra injected herself with a syringe of liquid, and then turned to Wolverine for his role. Moments later, Wolverine recovered and quickly broke free of his bonds, capturing the elderly woman. Though he had her at the mercy of his claws, Wolverine could not help but notice her smile, wondering what she thought was so funny.
Half a world away, the Mega-Sentinels arrived at their destination. With the civil war in Genosha having recently been resolved, 
the nation had become a homeland of sorts for mutants, with a population of over sixteen million people. Within minutes, the attack was complete, and half the world’s mutant population was no more. [New X-Men (1st series) #115]