アーツ オブ ブラック～魔女の箱庭～

Kyouya almost dies in an accident... and he is found by a witch, Satsuki. She looks at him and says, "Do you wanna be saved? However, you won't be healed completely." 

"I don't wanna die...," he nods. She saves him by using her magic. But his body needs to refill magical power regularly. Kyouya becomes her assistant in exchange of his life.

Now, Satsuki looks for witches, who ran away.
And Kyouya moves to a new school, where there is a rumor about witches...