The Undying is a slick, red figure in the shape of a humanoid. After completing Night 3, though before Night 4, an event surrounding Pirate Caverns will occur. During this event, Undying shows up in a cage on the bottom floor of these caverns after entering an elevator. It is currently unknown what its behavior is or what its role may be in the story.
The Undying is a red-colored figure in the form of a very tall, humanoid-like being with no face.
Undying starts out in its own room in a cage at the back, which is found on the bottom floor of the caverns. It then comes out of the cage and stares at the player while bending down. The rest of its movement pattern is unknown. Although, images in TBmHC and TGsEE hint that it will chase the player throughout the room. As of right now, Undying cannot be encountered in Pirate Caverns, as its area cannot be accessed.
It has been confirmed by the game creator that Undying will speak, and that its voice lines will be added in a future update.