Steel Darkness I: The Past

Four seniors go on a camping trips to find themselves stumbling upon a WWII bunker, but after they enter, they realise that they are not alone, and the last two groups of people who entered only had one survivor. Isaac and Marco split off from Grace and Cordelia to find a maintenance room to activate the power, but shortly after its activated, Grace and Cordelia find Isaac on the floor, breathing. Marco is nowhere to be seen, and above them is now an old Russian hospital...