Saikyou Onmyouji no Isekai Tenseiki ~Geboku no Youkaidomo ni Kurabete Monster ga Yowai Sugirundaga~

Haruyoshi Kuga's power as the strongest exorcist is all for naught when he is betrayed by his closest ally. With his dying breath, he casts the secret Spell of Reincarnation — when he next opens his eyes, he finds himself in a different world altogether, reincarnated as Seika Lamprogue, the son of a family of magic scholars! He quickly learns that the magic in this new world is quite unlike the sorcery he learned in his past life and that he can't use it. Fortunately for him, with his mastery of curses and the hordes of youkai at his disposal, he doesn't need to.