Kyo Kara Maoh! (2004)

Shibuya Yuuri, an ordinary high school boy got flushed in a toilet and he saw himself in another world. He was then declared as the 27th maou or the demon king of the Demon world by Gunter von Christ and Konrad Weller . When Yuuri now travels back to his new home (the castle), he meet up with Konrad's brothers. Wolfram von Biefield became Yuuri's fiancee when Yuuri slapped him for defending his mother's name. As a result, as the aristocrats tradition to propose a marriage, is by slapping one's left or right cheek with the back of your hand. By the time that Wolfram throw the knive and Yuuri picked it up, it means a battle between them. in their fight, Yuuri dodged Wolfram's majutsu and it hitted the girl just beside His Majesty. Yuuri got angry and turned into Maou, and punished Wolfram using his water majutsu. Yuuri fainted and when he came to senses after 3 days of sleeping, he did not remember anything that happened including using his powers.

Source: TMDb