Sekai Saikyou no Fukushuu Shinkan: Kami ni Tsukaeshimono, Maou no Chikara wo Te ni Ireru The Servant of God Gets the Power of the Demon King.

Synopsis

Raze was a prodigious priest who supported the legendary Hero. However, he was betrayed by the Hero with his party and sister was killed. He later absorbed the power of the Demon King and now seeks revenge on the Hero and the others that wronged him. The devastating revenge story of the priest who gained the power of the Demon King begins! (Source: Kadokawa, translated)