You can take this quest on the side as you adventure through the Underworld. It does not spawn very disasterous spawns like some of the other quests and it will open many quests as each Reaper freed will offer new quests and facilitate travel. You will need to travel to the location of each Reaper (a monument of Grenth) and defeat the group of three Terrorweb Dryders spawned by the quest. Once they are defeated, the Reaper of that specific area will appear.