Eimugai

One day, the high school girl Minon's beloved brother suddenly disappeared. All he left behind was an email note that he had been taken hostage by "A," a known serial-killer, and something about a mysterious place called "Eimugai." In order to get to that place, which did not appear in any searches, Minon tried to sneak into a suspicious building where she last saw her brother. However, it turned out to be the entrance to a forbidden place that must never be entered. This is a suspenseful search-and-rescue drama, as she entered a nest of "great evil" which will exceed all common sense and human understanding! (Source: Kodansha, translated)