This mod makes it able for you to summon an Ancient Frost Atronach, located at the the throne of Arch Curate Vyrthur (DAWNGUARD IS REQUIRED !) . This one is conjured by Arch Curate Vyrthur when he is sitting on his throne while the frozen falmer and chaurus attack you in Forgetten Vale in Dawnguard add-on.
its stronger than the normal Frost Atronach and its white instead of ice blue
You will have to be at the end of the Dawnguard DLC in order to be able to get it