Sing your sins! (Milgram x Project sekai)

Now then, sing your sins!
Ever since that night when a mysterious person came into the person, everyone felt uneasy. Not only that, Haruka, Shidou, and Mahiru have been presumed "dead" by the warden, much to everyone's dismay. As the prisoners fall into despair, they are then greeted by another warden, who goes by the name Ichi. Over there, they meet familiar faces, and learn things that they didn't expect to know nor see...