The Nightingales were scareactors that appeared in the Halloween Horror Nights 21 haunted house, Nightingales: Blood Prey. These ancient shape-shifting creatures are carnivorous banshees that prey on sick, dying or wounded warriors. Like carrion birds, they don't care who is evil, or who is good. As long as they feast on the weak warriors, they don't have to pick sides. During World War I, they took on the form of nurses in order to get closer to the soldiers. Due to the unprecedented scores of dead in that war, combined with the sheer chaos of trench warfare, they can afford to be choosy about who and what they eat, and are better able to maintain their nurse facades during lulls in the fighting.
They can be killed with enough violence however, hence why they prey on those who are too weakened to fight back, although they are opportunistic predators who will sometimes attack strong, healthy prey if they can ambush them without a struggle. They prefer to reveal their true forms and feed at night.