Uragirareta Touzoku, Kaitou Maou ni Natte Sekai wo Shouaku Suru
裏切られた盗賊、怪盗魔王になって世界を掌握する
A thief named Keith is coerced into joining a group to defeat the Demon King after his family is captured. His success in defeating the Demon King leads to betrayal and an attempt on his life by the King and the heroes. Surviving the attack, Keith discovers horns have grown from his head, transforming him into the "Phantom Thief Demon King," a being capable of stealing abilities and attributes from others. Empowered with the former Demon King's forces, including the "Four Heavenly Kings" and various monsters, he seeks vengeance against those who wronged him.