The thief Keith, whose family was captured, was forced to join the Demon King subjugation party. Thanks to his efforts, they managed to defeat the Demon King, but the king and heroes, who did not think highly of Keith, betrayed him and attempted to kill him. Barely escaping with his life, Keith found sinister horns growing from his head and transformed into the "Phantom Thief Demon King," an embodiment of destruction and violence who can steal others' skills and statuses! Along with the former Demon King's subordinates, the Four Heavenly Kings, and monsters under his command, he exacts vengeance on the humans who betrayed him!