In the Tower of Frozen Shadow: Part of the fifth encounter within the Tower of Frozen Shadow anniversary raid (see this quest entry for raid information).
In Karnor's Castle:
The Main items in his loot table are the Leggings for the various Kunark Class specific armor, as well as a line of two handed blunt staves for casters.
Background: Venril Sathir was the leader of the Iksar Empire who united the five tribes and brought them together to conquer most of the continent of Kunark.
Their rising power eventually attracted the attention of the Lords of the Ring of Scale (read: the Dragons of Veeshan's Peak). It was the mighty Trakanon who took it upon himself to wreak war apon the Iksar Empire, casting down this Necromancer leader.
Venril Sathir managed to cheat death somehow and now focuses on regaining the Iksar throne from his stronghold in Karnor's Castle.