Chronicles of Khaldun: Crux of Eternity
Suulkar is a primordial, still living yet imprisoned. During the Dawn War, Suulkar was defeated by the gods, and pinned to the bottom of the Sea of Howling Souls by a spear of ice that would later become the Frost Spire.
According to the tales, Suulkar was a monstrous beast, all tentacles and hate. It has potent psychic powers, and if the tales of Grannic are to be believed, it may have contacted the mind of Jarl Grugnurson.
The Shields of the Sorrowfell confirmed this fact, as Jarl Grugnurson was evidently attempting to revive the imprisoned primordial. It briefly stirred and tried to inhabit their minds before they disrupted the ritual and restored it to its fitful slumber.