Jujutsu Kaisen: A Thousand-Year Game

A modern soul transmigrates by accident and discovers that he has become Kenjaku, the Brain, sealed inside the Prison Realm. On the brink of losing consciousness, he draws on the mutations in his soul caused by transmigration and his thousand years of knowledge about cursed energy to force open a tiny crack in the seal. His soul's core takes over the body of an ordinary Curse User who has just died.