Uragirareta Touzoku, Kaitou Maou ni Natte Sekai wo Shouaku Suru

The thief Keith, whose family was captured, was coerced into joining the Demon King subjugation party. His relentless efforts contributed to the defeat of the Demon King. However, the king and heroes, who held Keith in low regard, betrayed him and tried to eliminate him. Barely escaping with his life, Keith discovered ominous horns sprouting from his head. He transformed into the Phantom Thief Demon King, an embodiment of destruction and violence capable of stealing others' skills and statuses! With the former Demon King's subordinates, the Four Heavenly Kings, and monsters under his command, he seeks vengeance on the humans who betrayed him!