To open this Demon Door you must bring a large audience to see him. A group of about ten people should do. Once you have your followers, fast travel to Brightwood Tower and use the Cullis Gate on top of the tower to warp to Wraithmarsh. Then, just walk up to The Demon door a short distance down the path and wait about two minutes for him to finish his speech. He will then open up and you may enter. The prize that awaits you is in an area called "Terry Cotter's Army" (a reference to the Terra Cotta Army). Go through the house and into the cave out back to discover a cave filled with statues (Terry Cotter's Army). Make your way through the statues to find a chest containing The Perferator, a legendary turret rifle.