http://www.newsweek.com/destiny-2-haunted-forest-guide-how-beat-nightmare-get-fragmented-soul-1175559

Destiny 2 ’s Festival of the Lost features the Haunted Forest, and the new PvE activity is the best way to accrue Fragmented Souls. This guide will help you slay Nightmares and get closer to earning the Horror Story Legendary Auto Rifle.

To unlock the Haunted Forest, visit Amanda Holliday at the Tower. She’ll send you to complete a few Lost Sector Missions on Io. When you finish them, talk to her again and she’ll give you a Masterwork Fighting Lion Exotic, if you don’t have one already. It drops relative to your Power level. After that, she’ll take you through your first run of the Haunted Forest. Subsequent runs are accessed from the Tower map.