Saikyou Onmyouji no Isekai Tenseiki: Geboku no Youkaidomo ni Kurabete Monster ga Yowai Sugirundaga @comic

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.

Haruyoshi (now Seika) is determined not to repeat his past mistakes in his second life by smartly laying low and avoiding flaunting his power. Yet accomplishing his goals and without drawing attention to himself proves to be easier said than done—will his powers be enough to live a happy life?!

(Source: J-Novel Club)