The Book of Lagier

What happens when a good-for-nothing human prince signs a contract with an all-powerful Demon King? Well, the prince dies instantly, and the Demon King is forced to take his place. But wait, there’s another catch! As the new Prince Lagier, the Demon King is obligated to fulfill the prince’s last wishes or never return to his former body. Join the Demon King and his demon friends as he gets in shape, battles the Angels, and races against time to complete his missions before it's too late.

(Source: Tapas)