Isekai De Tadahitori No Mamono Tsukai ~ Tenshoku Shitara Maou Ni Machigawa Remashita

Story:

After spending a lot of time searching, Noah eventually discovered the Sacred Book of Specialty Changes. Thanks to her power, the protagonist eventually succeeds in changing the Hero's specialty, which was until then. Either that's what he assumed. His specialty is now "Monster Trainer," about which no one here knows anything.