Karstaag can be fought as part of the unmarked quest Summoning Karstaag. It requires traveling to Glacial Cave and retrieving his skull. Once it has been obtained from the cave, it must be taken to Castle Karstaag Ruins and placed upon his throne. Several rieklings have taken up residence in the ruins and must be defeated first.
Once the skull is placed on the throne the ruins will rumble and begin to shake. A spectral Karstaag will appear from the throne and immediately attack. Once he has been defeated he will vanish, leaving only his ghostly remains.
As a reward for defeating Karstaag, the power Summon Karstaag will be granted which allows the Dragonborn to summon the beast to aid him or her in battle. This power is unique in that it can only be used three times, after which it is no longer available.
Karstaag is a powerful foe and can prove to be quite a challenge. He attacks much like a regular Frost Giant, using his club, fists and stomp attack, however these inflict much greater damage. His stomp attack is so strong that it can throw the Dragonborn across the room, much like an Unrelenting Force shout.He also has a permanent frost cloak with a large radius that inflicts frost damage, the severity of which is dependent upon game difficulty. Occasionally during Karstaag's stomp attack, a blizzard will begin to swirl in the courtyard, inflicting additional damage to health and stamina over time, which itself can be fatal. Because of this, some form of frost or magic resistance is useful, especially as stamina can be depleted within seconds.
Karstaag is completely immune to frost damage, poison and paralysis, however he does have a slight weakness to fire damage. Once he begins to take damage, he summons three Ice Wraiths to aid him in battle, if given the chance. He also regenerates health very quickly. During the entire battle the ground will shake violently, much like when staggered.