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https://finalfantasy.fandom.com/wiki/Jenova

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Due to the Jenova cells present in his body, the lifestream could not absorb Sephiroth completely. His will persisted and he learned everything about Jenova, the Cetra, and the circle of life, formulating a plan to be reborn as a god and rule the planet. Sephiroth's will overpowered Jenova's and he gained the ability to exert his influence on those infected with the creature's biomass and even turn them into vessels imitating his semblance, thus facilitating a plan to have them deliver the Black Materia to him, which would allow him to cast the ultimate black magic spell: Meteor.