Went Left, Mind Flayers

Both Shiki and Shirou's high schools share a senior trip destination. Unfortunately, that destination is into the home of a crazed magi and his demonic aberrations. When separated from the rest, Arihiko, Ayako, Shinji, Satsuki, and Issei must somehow band together to survive a land of horrors. Post-Ciel, Post-UBW.