Mahou Shoujo no Majo Saiban

What should have been the first day of high school instead leaves Ema waking in a dark cell on a remote island prison, far out at sea. An owl explains her predicament: she has been marked as a possible witch and a danger to the world, and is to spend the rest of her life imprisoned there. As she adjusts and gets to know the other girls accused alongside her, the owl gathers the captives with a chilling announcement, a witch trial will be held. One by one, the thirteen must single out a witch among themselves, voting for the next to be executed, and so a cruel game begins.