Koschei the Deathless

Koschei is an immortal incarnation of Death, and his presence in the world ensures that the cycle of life continues. He is associated with caves, eggs, tribal drumming and cave painting, ecstatic dance and mesmeric trance.
However, he is an entity without reserve, and as such if he were to be let loose upon the world there would be a tidal wave of mortality in the form of plague, war, or famine as he danced his dance of death throughout the lands. It was decreed he should be curtailed, and thus his soul was bound.
In some writings he is described as a Saint or Antipaladin of Poliel, the Plague Mother, but in all the histories he is presented as an embodiment of the scything force that counters exponential growth, and in every account he goes mad in some way and decimates populations to the brink of extermination.
And so, his gleeful and excessive soul was captured by Baba Yaga and hidden inside an egg inside a duck inside a hare inside a goat (called Martha), to whom she gifted eternal life, so that his ghoulish hunger for the living could be balanced with the generative capacity of the Great Mother.
It is believed his immortal body is imprisoned in an Azath House in some forgotten corner of Amarthaur.
Should Koschei's soul be returned to his body, there would be dire consequences for mortal beings, as the quiet contemplation of winter and the silent repose of death would be replaced by a dervish-like wave of carnage that would rival and stifle the celebratory growth and fruition of new life. The balance would be lost.
But there are those who covet the Soul of Koschei, either for his crazed and destructive power, or for his opposition to undeath, for Koschei abhors those who would deprive him of his harvest... and make the dead undead.
Now, where did that goat get to...?