Right click and use the Essence Culler from your inventory on one of the Sareth'Tal ghosts ( 90, 75, 180 ) /waypoint 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 ( 220, 90, 70 ) /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.