The Unknown is a mass of "something" that commonly moves around the Pizzeria.
Depending on where it is and at what time, the Unknown takes on many different appearances. It will commonly possess common objects in the Pizzeria and have them move on their own, like Masks and various other things. At 12AM, when not possessing anything, the Unknown presents itself as a slight aura that is barely noticeable. At 1AM, the aura makes snapping and crackling noises, like a thundercloud. At 2AM, it becomes more noticeable, as it takes the shape of white spirit of sorts with reddish eyes. At 3AM-4AM, it takes the appearance of a disfigured man, with many limbs missing. Finally, at 5AM, it takes the appearance of a rather tall being with many rows of teeth and eyes, as it slowly lurks around the restaurant.
Depending on the time, it will behave in different ways. When it is 12AM, all it may do is toy around with you by making different objects move in odd ways. At 1AM, it will do just the same, except it is able to disable cameras temporarily. At 2AM, it commonly stays around the animatronic, disabling the cameras where they are. At 3AM, it may slowly try to get to the Office, but never seems to get there. At 4AM, it can get into the Office and kill you, the way to scare it off is to shine a light in its direction, causing it to fade away. By 5AM, it goes on a rampage, attempting to get into the Office rather quickly, disabling cameras on the way. If the Unknown is looked at too much, it will instantly enter the Office and end your night (Only applies when it reaches its 5AM state). If the Unknown makes it to the Office without being looked at, the Freddy Mask must be put on, causing it to fade. (Only applies to 5AM state as well).