Keith, a thief, has his family taken prisoner and is forced into a party to defeat the Demon King. Thanks to his efforts, he manages to defeat the Demon King, but he is betrayed and nearly killed by the King and heroes, who are not happy with Keith. Keith barely escapes with his life, but before he knows it, evil horns have grown from his head and he has transformed into the "Phantom Thief Demon King", an incarnation of destruction and violence, able to steal everything from others, including their skills and stats! With the former Demon King's subordinates, the "Four Heavenly Kings", and monsters under his command, he will bring down the hammer on the humans who betrayed him!