Kostchtchie

Kostchtchie, known as the Prince of Wrath, was a demon lord and a patron of frost giants and rage.

Relationships
By establishing himself as a deity of frost giants, Kostchtchie directly encroached upon the domain of Thrym, though that titan did not take direct action against the demon lord. The dwarven deity Clangeddin Silverbeard considered Kostchtchie, as well as the other evil giant deities, a hated enemy.

Deeds
Around -160 DR, Kostchtchie's hortha was summoned by summoners of Narfell. Frost giants gathered under this hortha and fought on Narfell's side. It wasn't clear what exactly a hortha was, but given that urhortha meant avatar, chances were that hortha meant an aspect of the demon lord.