Quests:Dun Barethsol:The Misty Tower
The Misty Tower quest is given by Kidalethil, a ghostly Elf the player might encounter in the cemetery, in the woods, or even in the cellars of the Abbey of Shining Mist. She is mostly likely to encounter him on the edges of the swamp.
Kidalethil's request is simple: Remove the Usurper. Depending on Marcella's alignment and previous actions related to the other ghostly characters in the game, including Agdan, the bladespectre, and the Golem, (according to Kidalethil, the Golem is a ghost), Kidalethil will say more or less than that. Kidalethil can give some sense of the usurper's power and intent - even mentioning a wyrmstone - but he can't actually name the Usurper.
This quest is not essential to the plot, so if Kidalethil should not appear at all in a given game, that is alright.