Grimoire

A demon who uses the old magic and once drove the Kingdom of Saloum into the verge of collapse. After being sealed inside of a forbidden tome that was locked away in the kingdom's sealed archive for several centuries, he's released by Lloyd and they have their first encounter. But he becomes overwhelmed by Lloyd's immeasurable mana and sorcery and surrenders to him. After that, he turns into a cute familiar and serves Lloyd.

(Source: I Was Reincarnated as the 7th Prince so I Can Take My Time Perfecting My Magical Ability Official Site)