She tells you to go to the Somborn town in the Loping Plains and see Tanath L'Vec. He needs help finding an artifact. He sends you to retrieve the journal of another who died looking for the artifact (more of him later).
Go to Timorous Moors and find the 3 missing pages of his journal. They're on the beach. One to the East ( -146.86, -3.02, 475.8 ), one near a makeshift sailboat ( 110.08, -4.09, 381.15 ), and another in a keg in a sailboat. ( 24.12, -3.18, 332.26 )
Return to Tanath. He gives you the journal to read. It's called "A Passenger's Journal". It's written by Tanath's relative. Then talk to Tanath again.
He gives you instructions. Go get a device called an "Inert Essence Culler" from a chest west of the broken down spires. Take it to the spires and look up, There's a swirling power ball that you click to recharge the culler. Now its an "Essence Culler".
Use it to steal the soul of one of the gnolls in the valley to the south, just Northeast of the beach. When you do, the quest will update and you can evac away from the gnoll, if you don't want to fight it. You'll have an "essence of Sareth'Tal" in your inventory.
Go to 220, 88, 77 and release it. A spirit will run towards a spot there and it'll spawn the Dark Beacon. Harvest that. You'll have an "Ethereal Dark Beacon" in your inventory.
Return to Tanath. He plans to keep the Beacon and thus betray Xilania.
Go tell her. She knew he was a traitor and will deal with him later.