The Old Gods are deities who ruled, before Prythian was seen as a kingdom. They originated from a different dimension and 'fell' into the world. The magic of the Old Gods shifted to the High Fae, the Mother, and the Cauldron. These gods were soon forgotten after Prythian's formation, though a few individuals still worshipped them.
Koschei, Stryga, and the Bone Carver were three of the Old Gods. These creatures were death-gods, and were worshipped by ancient Fae. Stryga and Koschei loved the attention, though the Bone Carver liked the attention less. The Bone Carver feared his siblings, and allowed an ancient female Fae to lock Koschei in a lake on the Continent. While, Strya was confined to the Middle and the Bone Carver was locked in the Prison.