During the Halloween Event there is a chance for Haunted Doors to spawn in certain zones of Kryta throughout the world. These zones are mostly lower level zones and are usually included in part of the main event of this holiday season. The zones the doors can currently spawn in are; Queensdale, Gendarran Fields, and Kessex Hills. When you find a Haunted Door you have a single option, to knock on it for either a Trick or a Treat. You know how Halloween works by now, Trick or Treat!
If the door is a Trick it will spawn an undead "scary" type of mob for you to fight like a Skeleton, Mummy or a Zombie. This will trigger an event that everyone around you can join in on if they would like to. If the door is a Treat door it will shoot out a small closed bag which will contain a random goodie, most commonly a Trick-or-Treat Bag.
Added with Part 2 of the Halloween Event certain Haunted Doors will take you to the Mad King's Realm which is a level 80 area.