Gakuen Butou no Folklore

This is another Japan in a world supported by people with powers that derivate from fairy tales. Kouji was born with the power (‘cast’) “roi jewel”, which appoints the next King. He transferred into Gokyou Gakuen with his attendant Tsubomi, but due to living a secluded life because the country was raising him to be the next king, he and Tsubomi get lost and get to the school really late. When they arrive there's no one around, the go out looking for someone to help them and arrive in a arena where Kouji is attacked by three girls with powerful casts.

After the schools principal arrives and stops the fight she explains the Schools rules. The girl who defeat him in the Butoukai tournament and remain undefeated will marry him at his graduation, but Kouji and Tsubomi didn't know that and he will do his best to not lose to anyone and marry whoever he choses. They all ended up sharing a dormitory and he gradually began to know them better.

At that time, there was sequence of attacks on girls at the school almost every night. As they investigated the truth, they unexpectedly revealed the presence of a mastermind behind the incidents.