Cassandra Nova Xavier is a supervillain appearing in American comic books published by Marvel Comics, most commonly in association with the X-Men. Created by writer Grant Morrison and artist Frank Quitely, the character first appeared in New X-Men #114 (July 2001). She is a mummudrai, a parasitic life form born bodiless on the astral plane.

Cassandra is the twin sister of X-Men founder, the telepath Professor X (Charles Xavier). While in the womb together, Xavier recognized her evil presence and killed her, resulting in her stillbirth. The mummudrai that became Cassandra became telepathically entangled with Xavier, granting Cassandra some psionic powers herself, including the ability to exit the womb and create a body, with which she sought revenge on Xavier. As Xavier's ideological dark shadow who is bent on destruction and genocide, Nova is most infamous for being responsible for the massacre of 16 million mutants within the mutant homeland Genosha. In 2009, Cassandra Nova was ranked as IGN's 50th Greatest Comic Book Villain of All Time, the only villain introduced in the 21st century to make the list.

The character made her live-action debut in the Marvel Cinematic Universe (MCU) film Deadpool & Wolverine (2024), portrayed by Emma Corrin.

Cassandra Nova is a mummudrai, a parasitic entity who began life at the same time as Charles Xavier. Conceived without a body, Nova copied Xavier's DNA to make her own body, effectively making her his twin sister. She grew with her brother until she had fully formed hands and eyes, when she decided to try to kill Xavier by attempting to strangle him with his own umbilical cord. Xavier defended himself by hitting her with a psychic blast, which caused their mother to have a miscarriage, resulting in her physical body being stillborn. Nova survived as a mass of cells that clung to a sewer wall for decades, rebuilding her physical form and swearing revenge on Xavier.

Nova manipulates Donald Trask (a relative of Bolivar Trask) to utilize the Wild Sentinels to devastate Genosha, killing 16 million mutants. Nova duplicates Trask's DNA to gain control of the Wild Sentinels, then kills him.

Nova is captured by Cyclops and Wolverine and taken to the X-Mansion, where she escapes and defeats most of the X-Men easily. Nova uses Cerebra, an enhanced version of Cerebro, to switch bodies with Xavier. Xavier is trapped in Nova's body and unable to warn the X-Men before Nova shoots him.

Nova contacts the Shi'ar and manipulates them into battling the X-Men until they show the truth to the Imperial Guard. Angered at Nova's treachery and realizing the danger she poses, the Imperial Guard battle Nova, who defeats them and heads to the X-Mansion. She plans on using Cerebra to eliminate all mutants, but Jean Grey stops Nova and forces her out of Xavier's body. Nova's essence is tricked into entering the body of Stuff, a shapeshifting member of the Imperial Guard. There, she is locked in a self-repeating program.

Nova appears to return to her original form in Astonishing X-Men, alongside Sebastian Shaw of the Hellfire Club. Nova is actually an illusion created by Emma Frost, who is acting out post-hypnotic suggestions implanted by Nova prior to her being trapped in Stuff's body. Influenced by Nova and her own guilt over the Genoshan massacre, Frost created illusions of her younger self, Nova, Shaw, and Negasonic Teenage Warhead, a student of Frost who was killed in Genosha. Frost and the Hellfire Club attack the X-Men, only to be defeated by Cyclops and the Xavier Institute students.