Demon

Not to be confused with the Lesser Spirits.
The Demon is the evil entity which serves as the game's primary antagonist during the night shifts in The Mortuary Assistant. Every shift, one of the twelve demonic names will be randomly selected from the Sigils of Demonic Names which the player must reveal using Letting Strips. 
Once the name of the Demon has been revealed, a Mark must be built with the sigils spelling the demon's name, which should then be placed on the cadaver identified as the chosen vessel for the demon, which should then be burned in the retort. 
Should the player successfully burn the correct body with the correct demon name once, twice in a row, or fail to identify the correct body/demon, it will respectively reveal three out of the six endings. 
== Behavior ==
"At every moment, the demon is funneling its energy through you. Weakening you, until you are unable to resist." -Raymond Delver, Mini Cassette Tape 3 - The Process of Possesion
The Demon can first be seen should the player decide to leave the Mortuary after the first paranormal event during the first night shift. Upon entering the car, the Demon will manifest near the Mortuary entrance and, through paranormal whispers, will manipulate the player into staying and finishing the shift.
Once a shift starts, the possession process begins. The demon itself is not frequently seen during gameplay, however it makes its presence known by its paranormal interactions, which becomes more aggressive as the shift progresses.
During the first minutes of the shift, the player will occasionally see and hear paranormal activities, such as voices, door and window knocking sounds, light switches being turned off or flickering, and doors opening, closing or slamming shut.
As the shift progresses and possession gets worse, the demon begins causing hallucinations in the player. Some of these hallucinations involves Rebecca reliving past traumatic and painful moments in her life, while others involve trapping her in certain rooms of the Mortuary. The Demon also begins manipulating the cadavers, and as the Night Shift Database describes, these manipulations can be subtle or extreme. 
The Demon can manipulate any of the three cadavers, and while it appears to frequently manipulate the cadaver that the player is currently embalming, it can also manipulate cadavers in the cold storage, potentially confusing the player when identifying which cadaver is possessed.
The Demon will also employ the three Lesser Spirits, evil entities which will periodically haunt, stalk, startle and even attack the player in some cases, further worsening the possession progression.
As the shift comes close to the end and possession is nearly complete, the Demon becomes significantly more aggressive. It starts directly pushing and shoving the player against the floor or wall in certain parts of the Mortuary. It may use one of the cadavers to directly startle or even attack the player. It may also manipulate some of the items in the embalming room, bursting open drawers and cupboards by forcefully manipulating the items inside them. The Letting Strips also appear to aggravate the Demon even further, increasing the rate that the possession progresses. 
It is during this phase of the shift that the Demon may sometimes be seen: it can peek from the walls while the player is carrying the last embalmed body back to the storage room. Additionally, there is a chance for the Demon to manifest and forcefully push the player back after having its four sigils being revealed by the letting strips.
== Ending The Shift ==
Once the gurney with an embalmed body is placed by the retort and it is activated, regardless of whether it is the possessed body or not, the Demon can be heard taunting and laughing as the body is inserted into the retort. Once the retort starts burning, thumping and screaming sounds can be heard from inside the retort. Eventually, an explosion occurs, knocking the player down, while also destroying the gurney and the retort hatch. The Demon can be seen crawling out of the retort towards the player, only to be dragged back into the retort as the player's vision fades, subsequently revealing one of the game's possible endings.