Depicted as normal humanoid figures wearing doll masks and black clothing, The Dolls roughly resemble a family comprised of an older woman in a corporate suit, an older man in a business suit, a blonde pigtailed girl in a black dress, and a hooded boy. They are cruel and sadistic killers, partial to torture by fire and they appear to be deliberately silent, never uttering a single word. They are not seemingly supernatural in appearance or nature.
A mask resembling one worn by one of the monsters is seen in when Dana first enters the cellar and it's probably what summons them.
The Dolls are first seen in their cube, barely glimpsed during the cube scene. Later, the Woman and Hooded Boy are seen on a mini monitor while Sitterson was looking for Marty and Dana. During the System Purge, The Dolls are seen in the second wave of monsters exiting the elevator during the Purge. They can also be seen in a camera view angle binding and gagging Facility workers together with tape before dousing them in gasoline and setting them on fire.
Most likely a reference to the antagonists of the 2008 horror flick, The Strangers.