Warning! Spoilers for Heroes Reborn: Magneto and the Mutant Force #1!
One of the Hyperion and the Power Princess halfway across the galaxy.
Created by Grant Morrison and Frank Quitely in New X-Men #114, Cassandra Nova is Charles Xavier's darkest secret, what the Shi'ar called " a mummudrai," someone or something that is a person's spiritual opposite and equal. While Cassandra gestated inside their mother's womb, an infant Charles Xavier's power was great enough that he sensed her great evil and fought back, preventing her physical birth. But Cassandra's powers enabled her to slowly grow a body and develop her own impressive powers. She orchestrated the events of the infamous Genoshan massacre that killed sixteen million mutants and continued her reign of terror when she mentally occupied her brother's body and wreaked havoc on his personal and professional life. Although the X-Men were able to defeat and capture her, Cassandra continued to return, either as a mental manifestation in Emma Frost's mind or inciting international incidents by infecting people with minuscule Sentinels that would send them into a violent rage in the vicinity of mutants. She was last seen getting a dose of her own medicine when Jean Grey installed a Sentinite in Cassandra's mind, causing her to feel guilt and regret for her actions as a result of experiencing empathy for the first time ever.
In Heroes Reborn: Magneto and the Mutant Force #1 by Steve Orlando and Bernard Chang, the Marvel universe has been reimagined to a world where the Avengers were never created, leaving a world where mutants are hated, feared, and oppressed more than ever. Years after the Mutant Massacre that saw the death of Charles Xavier, Magneto believes that Charles has reached out to him and convinces of the X-Men to him on a rescue mission. When Magneto is the last man standing while the others deal with an attack by Hyperion and Power Princess, he learns that this entire situation is actually a meticulously planned trap. The entity posing as Charles its to it all and uses Magneto's body to help it escape, using its long gestating powers to make the Squadron Supreme disappear before making its grand entrance as the X-Men's new leader, Cassandra Nova.
It's been established that the he is neither invincible nor incapable of defeat.
The reintroduction of Cassandra Nova is a clever twist that falls within her pattern of manipulating others for her own agenda and seeking to destroy any image of her heroic sibling. She is the dark reflection of this new reality's expectation for its mutant heroes, creating a darker, less restrained X-Men prepared to survive by any cost. Although some may disagree or contest Cassandra Nova's new leadership, they'll likely fall in considering her dangerous display of power against one of the Heroes Reborn universe's biggest threats.
