Let me guess, two people and a creature of some sort, why not turn the story from the average, they survive, to they know their going to die so they live their lives for three days then die on the forth... I think that's pretty demented... The creature toys with them, gives them blood written messages, horrible hallucinations, chases them and forces them to barricade thereselves in an abandoned military facility, then it activates traps, they can live for three days then either die by traps or become its dinner