Yuuri Shibuya! I AM the Demon King!

When Yuuri wakes up after his first time using magic he finds out from Wolfram that Conrad and Gwendal have gone off to protect a human refugee village that is being attacked. Yuuri convinces Wolfram to take him out to help. On the way they run into Adalbert who tells Yuuri that it is humans that are attacking the town so it is a fight between humans. He tries to convince Yuuri to come with him and leave the demons but Yuuri decides he is going to help the demons. Yuuri decides to become the demon king and when they return to the capital they hold a coronation ceremony. To complete the ceremony he has to stick his hand in a waterfall but when he does he gets sucked back to Earth.