Right click the wisp that spawns above your head and empower the Essence Culler. This will not remove your invis.
Right click and use the Essence Culler from your inventory on one of the Sareth'Tal ghosts (loc: 90, 75, 180). This will remove your invis. You will receive an Essence of Sareth'Tal.
Use the Essence of Sareth'Tal from your inventory to release the spirit near (loc: 220, 90, 70) and follow it. This will remove your invis.
Pick up the Dark Beacon that spawns on the ground where the spirit disappears. Taking the beacon will NOT remove your invis!
[Note: The Essence of Sareth'Tal has only one charge. If you are unable to locate the Dark Beacon on the first attempt, use the Essence Culler on another Sareth'Tal spirit and retry.]
Return to Tanath.
Return to Xilania Nevagon to complete the quest and claim your reward.