Gokuin no Sentou

Taking an ordinary tour of his sister’s workplace, Mitsuya Harunobu had expected the day to be friendly and amusing – not a desperate struggle for survival. The voice that echoed through his head was cold and malicious, "Only one with the strongest desire shall earn freedom." In the passing of a single moment, the gleaming corporate tower was reborn into a sadistic infernal prison: an unknown malevolent force had seized control of the building and corrupted nearly every person within. Rape. Torture. Murder. These were now normality. Somehow unaffected by the supernatural entity, Harunobu resolved his sister would escape the incomprehensible nightmare unharmed - even if Harunobu would have to sacrifice himself in the process.[From Eroge Review]