Dolls

The Dolls are a group of Monsters imprisoned in the Facility. They are listed on the whiteboard, having been bet on by the Kitchen Staff. They can be seen twice during the System Purge, upon their release during the second wave and on monitor.

Physical Descriptions and Characteristics
The "Dolls" are a group of seemingly normal humans wearing all-black attire and white, expressionless masks with black lips and thin eyebrows. They are not supernatural in nature or appearance.

The Dolls seem to roughly resemble a familial unit, consisting of an older woman and man in plain black business suits, a younger woman  with pigtails, black gloves, and a short black dress, and a younger man in a black zip-up hoodie and black jeans. The younger male Doll is shown to carry a switchblade, although it does not use the weapon during the System Purge.

There is nothing outwardly supernatural about the Dolls. However, like all other "normal" monsters in the Facility, their indefinite dormancy in their cube between summonings belies this mundane appearance. Their calm, methodical movements and total lack of emotion or communication gives them a cold and sociopathic demeanor. The Dolls are shown to cooperate and interact with each other to trap and kill prey, adding to their lethality.

The Dolls are shown to use materials like duct tape, gasoline tanks, and matches to kill their victims. Since items like gasoline tanks would not be readily available in an office setting like the Facility, it is possible that the Dolls may have the ability to manifest certain tools in pursuit of killing, but this is unconfirmed.

The Dolls are summoned by a mask in the cellar, nearly identical to their own, that lies amongst a pile of various dolls and other masks.

Appearances
The Dolls are first seen in their cube, barely out of view during the cube scene where the camera pans-out. Later, the older female and younger male 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 their elevator quietly. 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, quietly watching them burn to death.

Inspiration
The Dolls are based largely on mask-wearing slasher film villains who show little emotion and relentlessly hunt down and kill their victims, such as Michael Myers from the Halloween franchise. Since the Dolls work as a group rather than individually, they most closely resemble the antagonists of the 2008 horror flick, The Strangers, the masked home invaders in the 2011 slasher film You're Next, and similar films. It is also possible that the Dolls take inspiration from The Purge, where a group of masked children target a man's house and his family. In fact, the premise of The Purge franchise somewhat resembles that of Cabin in the Woods in that there is a condoned, ritualistic, nationwide day of mass anarchy and violence that enables society to live in relative peace for the rest of the year.