The Festival of the Lost is Destiny 2's Halloween-themed seasonal event and one of its longest-running seasonal events. It includes in-game trick-or-treating, masks to dress up as your favorite characters, and unique in-game and cosmetic awards. Starting with the second year of Destiny 2, it also added a unique activity: the Haunted Forest.
If you've spent time on Mercury as part of Destiny 2's Curse of Osiris expansion, you've been in the Vex's Infinite Forest. The Festival of the Lost reimagines the location a great deal darker and spookier, with tougher enemies and bosses to defeat along the way. Fighting your way through the Forest earns you candy and Chocolate Strange Coins, which you can trade with Eva Levante in the Tower for exclusive Festival rewards.
Your goal in the Haunted Forest is to get as far as you can, but that's not dictated by distance. Instead, you're trying to clear "branches" of the Forest by defeating enemies. That means you want to kill baddies as fast as you can, but it doesn't necessarily mean you need to keep moving forward quickly to do so. The more enemies you kill and branches you clear, the better your rewards.
Check out the video above to see a run of the Haunted Forest and all the terrors you'll need to worry about therein.