Mahou Shoujo no Majo Saiban

"Witches lurk among these girls." It was supposed to be Ema's first day of high school. But instead of waking up in her bed, she found herself in a dark prison cell. She found herself suddenly trapped in an island prison, isolated in some unknown, distant sea. In her confusion, an owl informed Ema of her new situation: "You have been identified as a potential witch — a threat to the world. Thus, you will live here as a prisoner for the rest of your life." The life Ema had taken for granted had suddenly collapsed. As she began her new life as a prisoner, Ema got to know the other girls in her situation. Her fellow witch candidates. However, one day, the owl gathered the prisoners for a shocking announcement: "We will be holding a witch trial." One by one, the girls would have to single out a witch among the thirteen of them... In other words, they would have to vote out the next candidate to be executed. It was the beginning of a cruel game indeed... (Source: Steam)