Hitokui: Origin

There is a rash of sudden group deaths, where all members die of a heart attack. These people receive a chain mail and are then drawn into the lair of a man-eating monster, a "Hitokui." Generally, only the last person standing survives, with the others dying of a heart attack in the real world when they are eaten by the monster. High school student Youta is drawn into one of these lairs, which doesn't go quite as the monster planned. He survives, and retains his memories of what happened there. However, it looks like the world of man-eating monsters is not finished with him yet...

(Source: MangaHelpers)