Although the Reaper of the Labyrinth is credited as quest-giver in your quest log, it is the Lost Soul next to where you start in the Underworld, from whom you can obtain this quest. As soon as you accept this quest three groups of Grasping Darknesses will spawn (one group of four to the south-west and two groups of three each to the north). It will take a short while until the last group moves to the chamber. The Darknesses are not very dangerous, but can be difficult enemies if you try fighting several groups at once. If you haven't cleared the area of the Bladed Aatxes, you will have a serious problem, so it is best to first clear the nearby area before you take this quest. This is where many first time visitors make a mistake as they instantly rush to take any quests.