The Haunted Forest is a variation of the Infinite Forest that debuted in the Festival of the Lost in Destiny 2: Forsaken. During the event, the Infinite Forest begins to exhibit strange behavior, with the primary simulation area within Mercury's core becoming blackened and overrun with entities known as Nightmares.
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The area can be formally accessed after accepting the primary Festival of the Lost quest from Amanda Holliday. Travel to Io and clear out two Lost Sectors full of Taken enemies. The readings from the areas pinpoint drastic changes within the Infinite Forest. After clearing the two Lost Sectors, Guardians can warp to the Forest via the Destinations map of The Tower.