Hall of Judgment
The Hall of Judgment, sometimes known as "Dhuum's chamber", is the name of an area in UW. This is where the fight with Dhuum takes place after completing the first 10 quests, and where the Underworld Chest spawns after defeating Dhuum.
The Nightman Cometh
This quest becomes available from King Frozenwind as soon as the other 10 quests have been completed. You do not need to have taken or accepted the reward for any of those quests in order to take this one. As soon as the quest is taken, the door leading into the inner part of Dhuum's chamber is opened, Dhuum slowly moves to the centre of the chamber, and all of the Reapers de-spawn and re-spawn inside the chamber. If a Reaper is in aggro, it will not de-spawn, but a duplicate will still spawn inside the chamber.
Entering the room places the Curse of Dhuum effect on you. This prevents you from casting res skills (except BUH), although scrolls can be used. If you die, you lose the effect, take on Spirit Form, and are teleported to vale. Once the entire party is inside the chamber, the door will close again, and Dhuum will turn hostile. The door to vale will also close, and Mayor Aligheri will now have a dialog offering to teleport you back to Dhuum's chamber.
When Dhuum's Rest is filled and Dhuum's health is below 25%, he will turn non-hostile again; the Underworld has been completed. The door will open, King Frozenwind will come inside, and the Underworld Chest will spawn.