He is a unique Frost Giant, who originally dwelt in Castle Karstaag, on Solstheim, with his serving tribe of Rieklings. He and the Skaal tribe have lived in peace for many years.  During the events of Bloodmoon, the Skaal have not heard from the giant for some time. It is revealed during the main quest that, Karstaag was captured by Hircine for his hunt.
Karstaag has the following abilities:
- 50% Reflect
- 100% Resist Frost
- 50% Resist Magicka
- 100% Resist Normal Weapons
- 100% Resist Paralysis
- Restore Health 4 points
- Icy Blood — Restore Health 5 points
- He is the only frost giant encountered in The Elder Scrolls III: Morrowind and both its expansions.
- 200 years later, the Dragonborn may battle the spirit of Karstaag in the unmarked quest "Summoning Karstaag" by placing his skull upon his throne in the ruins of his castle.