Imprisoned Dvergr has 1,500 health and spawns in the Deep North.
Spirit does nothing to it at all. Blunt, slash and pierce land at full damage, so that is what to bring.
Its hardest hit is dverger arbalest shoot deep north at 250 pierce, out of 3 attacks.
Of everything craftable, 
Frostfire Crossbow, 
Thunderblood Crossbow and 
Lightning Strike put the most damage through what it resists.
It always drops 
Coins.
Imprisoned Dvergr is a Dvergr faction captive found only in Mörkhalla, up to seven of them across the dungeon's 11 rooms. It has 1,500 health, is immune to spirit, and shoots a Deep North arbalest for 250 pierce, above the 210 an Ashlands rogue manages, with a 140 blunt bash to match. It always drops 10 to 20 Coins, and rolls each of the four ancient gemstones separately at 10%: 
Draumyx, 
Grimvarn, 
Solryth and 
Veydris.
They have no resistances beyond the usual spirit immunity, so damage is not the problem. 250 pierce through a corridor is, and no shield in the game absorbs that comfortably, so break line of sight and close the distance rather than trading. In melee they drop to 140 blunt, which is survivable. Each one carries 10 to 20 Coins and a 10 percent chance at each of the four ancient gemstones: Draumyx, Grimvarn, Solryth and Veydris.
With devcommands enabled, this spawns Imprisoned Dvergr in front of you. The third number is the level: 3 spawns a 2-star.
Creature ID: DvergerDeepNorth · all console commands and IDs