The Four Fiends of Niflheim

A group of people gather in a mansion built in the middle of a frozen lake. Throughout the night, they are picked off one by one in a series of impossible murders. A man is killed in a room filled with people -- and yet, none of them could have done it. Another is murdered by a killer that vanishes through a door nobody could've gone through. The murders continue, escalating in brutality. The enemy is among them. But who is it? What are their motivations? What secrets are the guests hiding?