Retrieve the Essence Culler from a clickable chest in a wagon at ( 150, 45, -330 ) Copy/waypoint 150, 45, -330 its near the spire. This will remove your invis.
Step into the center of the Reconstructed Ulteran Spire, then activate the wisp that spawns above the portal.
Right click and use the Essence Culler from your inventory on one of the Sareth'Tal ghosts ( 90, 75, 180 ) Copy/waypoint 90, 75, 180 they are near the area outside the crypt of valdoon. 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 ( 220, 90, 70 ) Copy/waypoint 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. This will remove your invis.
Return to Tanath.
Return to Xilania Nevagon in Greater Faydark to complete the quest and claim your reward.