Hitokui: Origin  

ヒトクイ-origin-

A group of individuals are dying suddenly from heart attacks after receiving a chain mail message. They are then transported to a lair where they encounter a "Hitokui," a monster that feeds on humans. Typically, only the last person surviving escapes, while others die in real life when consumed by the monster. In this case, high school student Youta is one of those who enters a lair but manages to survive. Despite retaining his memories of the event, it seems that the world of man-eating monsters is not done with him yet.