Use the Potion in your inventory. Once you turn red (invisible) walk around until you see NPCs. Stay as far away from them as possible and continuously click "continue to eavesdrop" until you are discovered.
The NPCs turn aggro; kill them all. Once a vessel is dead an amulet drops on the floor and the quest updates.
Click the amulet on the floor. The quest updates again.
A cutscene called "An Out of Body Experience" plays in which you see an army of shadow brutes and shadowed men surrounding a hooded woman on a pillar of rock. Your own character can be seen floating to one side of the rock.
Return to Evdokia. (You can use the Neriak carpet on the dock ( 19, 3, 285 ) Copy/waypoint 19, 3, 285 to go to Sinking Sands and from there to Gorowyn.)