There are several components to this, including new things to buy at the Eververse Trading Company, the recently added in-game store. There are three new emotes--Boo (300 Silver), Monster Dance (500 Silver), and Zombie Dance (700 Silver, available only on Xbox One/PS4)--and the option to buy a mystery bag called the Treasures of the Lost.

You'll receive one of these mystery bags upon embarking on the new Halloween questline at the Tower. The bag is guaranteed to include a random legendary mask (you can see a bunch of these in the gallery below), as well as a chance for various consumable items that have different effects, such as providing extra Glimmer or speeding up weapon upgrades.

The questline consists of several parts. Initially, you'll be tasked with collecting candy from various NPCs around the Tower (except for Eris, who has a rather Eris-like item for you instead). After doing this, you'll receive a few more quests, tasking you with doing things while wearing one of the new masks. This includes taking part in public events, collecting engrams, and killing other players in Crucible.

There's other stuff to check out as well, including the redecorated Tower (accompanied by new sound effects, like screams and wolf howls). Also be sure to take a look at the mask Cayde is wearing (or see it in the gallery above), and check with the Postmaster for a consumable item that changes your respawn animation.