Cassandra Nova is a fictional 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, Cassandra first appeared in New X-Men #114 (July 2001). Cassandra is a "mummudrai," a parasitic life form born bodiless on the astral plane. The mummudrai that became Cassandra became telepathically entangled with the future Charles Xavier, who possesses vast mutant telepathic powers.
The Great White Owl
On Earth-TRN342, Cassandra Nova has somehow survived for four thousand years and nearly conquered the world as the Great White Owl after she unleashed the Great Corruption and dropped entirely the Veil that separated the Main Reality with the reality of the Revenants, better known as the Mummundrai.  As the Revenants were unleashed on Earth, Cassandra becomes their Queen and soon enough Bishop's presence in this reality came to Cassandra's attention. Bishop had been flung into this future Earth after failing in his attempt to kill Hope Summers and soon became a Revenant Hunter, even adopting a little girl named Amber. While on a mission to track a Bull Revenant called the Demon Bear for two months, Bishop and Amber found it in the forests outside a human settlement. Little did they realize that the Demon Bear was part of a trap set by the Great White Owl herself. Bear and Owl attacked, and Bishop held them off long enough for Amber to flee to safety, but ended up being possessed by both Revenants for his heroism. The Revenant Queen intended to use Bishop as a vessel, for she wanted to travel back in time to set off the Great Corruption several thousand years earlier than it originally happened.  Re-materializing in the present outside Union Station in Los Angeles, California, the Owl Queen directed Bishop's body towards finding a worthy psychic to sacrifice for the Corruption's rituals. She zeroed in on a new mutant named Ginny Guzman, but the girl was already being fought over by Spiral and a team from the Jean Grey School. The Revenant Queen allowed the Demon Bear to run roughshod over the girl's defenders until she got close enough to project her own corrupted psyche into the child. She left behind a very rattled and confused Bishop and Demon Bear, who were knocked cold by Puck.  Eventually Cassandra took possession over the body of Ginny and killed another new mutant psychic to begin the Great Corruption. As the Revenants began to be unleashed by the hundreds throughout Los Angeles, Bishop and his allies attacked the Owl Queen at her base at the Griffin Observatory and, through a complicated sequence of events, trapped Cassandra in the humanized body of Psylocke's Revenant counterpart, and then sacrificed her in a ritual that reversed the Great Corruption before it fully dropped the Veil to the Underworld.

Q: What is the great white owl?
A: and nearly conquered the world as the Great White Owl

Q: who nearly conquered the world?
A: Cassandra Nova
Was Cassandra the Great white owl?
Cassandra Nova has somehow survived for four thousand years and nearly conquered the world as the Great White Owl
Cassandra Nova has somehow survived for four thousand years and nearly conquered the world as the Great White Owl