Received from: Automatically after talking with Dethmold during Conspiracy Theory.
Step 1: find the Dun Banner's standard, Vandergrift's sword and a piece of Seltkirk's armor. Lastly, acquire Henselt's medallion
In order to continue, you have to complete Courage Symbolized, Death Symbolized, Hatred Symbolized and Faith Symbolized.
Step 2: Talk to Dethmold about the curse
After collecting all the items, talk to Dethmold (M25, 7) once you're prepared and tell him you're ready to lift the curse.
Step 3: Take part in the ghostly battle within the mist
The ghostly battle is identical as the one in Iorweth's path. A detailed description of the mission can be found in [ Act 2 - Iorweth - main missions ] The Eternal Battle.
Right after taking off the mist curse, you will have another flashback connected with Returning Memories.
You will unlock Vergen Besieged.