Mahou Shoujo no Majo Saiban

"There is a girl among you who has become a witch."

On the morning she was supposed to start her first year of high school, Ema woke up inside a dimly lit cage. Imprisoned within a remote island prison mansion for a crime she doesn't remember committing, a confused Ema receives a declaration from a single owl:

"You have been identified as a potential witch — one who brings harm to this world."
"You will spend the rest of your life here as a prisoner."

The everyday life she took for granted completely collapsed. As Ema gets to know the other girls, all fellow witch candidates, during her imprisonment, the owl eventually gathers all the prisoners and declares:

"A witch trial will now commence."

And so began an utterly cruel game — a process of selecting, one by one from the thirteen girls, the witch... that is, the one to be executed.

[From Steam]