The Bone Witch in Njorndar Village wants you to possess a Water Terror and use it to slay 10 vrykul in the greater Jotunheim region.
Qualifying vrykul are Jotunheim Sleep-Watchers, Jotunheim Warriors, Mjordin Water Magi, Njorndar Spear-Sisters, and Valhalas Vargul.
The water lock outside my cave is teeming with elementals that the vrykul magi have yet to tame. I am going to teach you a trick for possessing these creatures for a short time, and you will use them to wreak havoc upon Jotunheim!
<The banshee-infested vrykul draws magic in the air and touches a large finger to your forehead.>
Go now and open the gate to the lock. A water terror will rush out and you will possess it to destroy our enemies!