Seika is the isekai reincarnation of Haruyoshi Kuga, who is said to be the strongest onmyouji (yin-and-yang master) in history, using the secret technique "Curse of Reincarnation" that he devised himself. Learning from the mistakes that led to his initial demise, he vows to live his new life inconspicuously, cunningly, and happily.

(Source: The Reincarnation of the Strongest Exorcist Wiki)