Completing the second sequence of Summoning Sciences will summon this creature. It can be pickpocketed for a minor item. However, after it is summoned it goes on to kill travelling merchants. The Warden can stop it by completing Unintended Consequences, a Chanter's Board quest. At the beginning of the fight, the Whim will spawn 4 images of itself, of Normal rank. If your party can hold the various targets, this is a relatively straight forward battle.
DropsMagus War Boots
Drakeskin (Tier 7)