If the Ancient Wardstone atop Harlan's Lament is active and possessed by the Defiant you will receive this quest the moment you enter Harlan's Lament. It does not matter if someone else has already started it. Go help, Defiant!
In the tar pits you will see wisps that cannot be targeted. Move into one to become a wisp. You are now non-KOS to all the mobs! Move to one of the the teleport rings at the base of the ramps up to the first level of Harlan's Lament to be teleported directly to the ledge where the Ancient Wardstone is, then move to the wardstone to transfer the energy. now step into the teleport ring there to be returned to the ground!
This is a cooperative task. All Defiant in the area can help, and all get the quest reward when complete. Click on the diamond in the top-center of your screen to Finish the quest.