Summoning Karstaag

Walkthrough
This quest is not tracked by the game, so the Dragonborn might accidentally stumble into it after collecting Karstaag's Skull from the far wall of the Glacial Cave.

Reach the Courtyard in the Castle Karstaag Ruins (either by directly lockpicking the courtyard's gate or via the Castle Karstaag Caverns), approach and activate the large throne of ice, Karstaag's Throne, and the Karstaag's Skull will automatically be placed there.

This summons Karstaag a very powerful and very dangerous frost giant. Overcoming him yields a new Summon Karstaag power, letting the Dragonborn summon the beast to help in combat. But this is only possible three times.

Karstaag beschwören (Aufgabe)