The Haunting Shadow is an antagonist that appears throughout the game. The player often sees the Shadow in dreams and in cutscenes. The Shadow also appears several times during normal gameplay. In both the Fanciful Forest and Sweaty Jungle biomes, the Haunting Shadow will occasionally appear and chase the player. If the Shadow catches the player, the player will receive the Poison effect.
The 'ghost' version of the player tells you that the Shadow is the creation of another's mind.