From DrCoke at MangaHelpers:
One day, high schooler Nishiura Kenichi was tasked to check if his classmate, Kurama Shion, was still alive. Shion has not shown up to her classes in the past two months, and the teachers were getting worried. When Kenichi arrives at her place, she decides that he would make a good test subject for her recently acquired black magic powers. With her black magic curse, she turns Kenichi into her personal slave. The life of Shion, the master of the black arts destined to take over the world(?), and her loyal slave Kenichi begins here!
