Stennbrun is an Elite Sturmbeorn Blacksmith who wields a huge lightning mace, and can be found in the Temple Steppes with his giant fire sword-swinging buddy Gunnbren. Both smiths spawn in the smithy in the Beast Warrens where you find the Armor Schematics associated with The Warbeast Armory side-quest.
Note that you do not have to be on the quest to get the smiths to spawn - they always spawn in the smithy in the Temple Steppes.
Unique Gem Drop[]
Stennbrun drops The Eye of Stennbrun, a unique level 13 Gems (T2) which confers a +15% to ice damage when applied to armor or trinkets, and +15 ice damage when applied to weapons. Note that the Eye requires a minimum Item level of 5 to use. The high ice damage percentage makes it a good choice for twinking items for a Berserker or Embermage focusing on Frost-based skills.