Gokuin no Sentou by Black Cyc - Adult Sex Game

Taking a routine tour of his sister's workplace, Mitsuya Harunobu expected the day to be friendly and funny - not a desperate struggle to survive. The voice that echoed in his head was cold and angry, "Only the one with the strongest desire is to earn freedom." In the passage of one instant, the gleaming corporate tower was reborn into a sadistic hellish prison: unknowably malevolent forces took control of the building and damaged almost human inside. They were now normality. Somehow unaffected by a supernatural entity, Harunobu's determined sister would be able to escape the incomprehensible nightmares safe and sound - even if Harunobu would sacrifice himself in the process.