Kyo Kara Maoh! - Season 1 Episode 3

When Yuri 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. Yuri convinces Wolfram to take him out to help. On the way they run into Adalbert who tells Yuri that it is humans that are attacking the town so it is a fight between humans. He tries to convince Yuri to come with him and leave the demons but Yuri decides he is going to help the demons. Yuri 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.