The Labyrinth, abbreviated as "lab", is the name of an area in UW. The team spawns in lab when entering UW. Several quests take place here: Clear the Chamber, Unwanted Guests, Escort of Souls, and Restoring Grenth's Monuments.
Clear the Chamber
The first quest, given by the Lost Soul near the initial spawn point, comes in two parts. Taking the quest spawns 3 groups of Grasping Darknesses: one group of 4, and two groups of three. Killing all of these opens the 4 gates leading out of the chamber, giving access to the final objective: kill the 3 Terrorweb Dryders around the Monument. This completes the quest, spawning the Reaper of the Labyrinth, and allowing you to take any of the next 3 quests.
Restoring Grenth's Monuments
This quest, known as "Restore", spawns a group of 3 Terrorwebs at each of the 6 other monuments around UW. Killing a group spawns another Reaper at that monument; spawn all the Reapers to complete the quest.
Escort of Souls
Taking this quest, which is abbreviated to "Escort", spawns a group of 6 allied New Souls near the Reaper, and 4 groups of Mindblade Spectres along the way to Vale. The Souls will walk very slowly to the bottom of the stairs, and wait there until the first 2 groups of Mindblades are all dead, after which they will walk through the basement and half way up the next flight of stairs, until all the Mindblades are dead. They will then walk all the way to Mayor Aligheri. The Souls can be attacked by all enemies, but do not use any skills, attack, or kite to defend themselves, and if any one of them dies, the party will fail. When the first Soul reaches the Mayor, the quest completes, but each Soul does not become invulnerable until it reaches the Mayor, so you still need to protect the Souls until they all leave the party list.
Although the Souls can walk through allies (such as the party), it's very common for them to get stuck on the hostile Tortured Spirits in Vale.
Taking this quest, which is abbreviated to "UWG", spawns 6 Keepers of Souls all around Lab, and each Keeper comes with 2 patrols of 3 Vengeful Aatxes, 2 groups of 3 Terrorwebs, and 3 Skeletons of Dhuum (except the final Keeper, which only has 1 skele [Verification required]). Killing each Keeper de-spawns the associated Vengefuls and Terrorwebs, but the Skeles remain. Killing all 6 Keepers completes the quest.
The initial area the party spawns in is known as the "chamber". 4 gates lead out of here: 2 go deeper into lab, towards vale, and the others lead outwards towards wastes and mountains. The area downstairs from the Monument, leading towards vale, is known as the "basement".
Most of the enemies in lab are nightmares: Bladed Aatxes, Grasping Darknesses, and Dying Nightmares. Most Graspings are patrols, but there are 3 groups pop-ups in the chamber, around the level just above the spawn point. Above and below the stairs leading from the basement to vale are pop-up groups of 3 Mindblade Spectres, each triggered by walking past their spawn point. Apart from the 3 dryders at the Monument, the only other enemies are Skeles patrolling around. Many more enemies spawn when Restore and Escort are taken.