Your forces were out on a routine patrol and the undead were suddenly everywhere! Your forces were badly depleted in the conflict thus you must continue to do your job and keep the undead population under control until a solution can be found.
It's a nightmare. You are trapped in an abandoned mine with an injured comrade at your side against God-knows-what. Make your stand, get out alive. Three words of advice: shoot to kill. These things are not human, they are monsters..