Koshchei, often referred to as Koshchei the Deathless or Koshchei the Devil, is a figure from Slavic folklore. In the Hellboy Universe he is the slave of the Baba Yaga. He is quite literally 'Deathless' as he cannot be killed because he put his soul inside a duck, inside a rabbit, inside a goat. Baba Yaga is said to have 'hidden the goat' and holds it hostage so Koshchi will do her bidding, in hope that the Baba Yaga will one day release his soul and allow him to die, as Koshchei has become tired of immortality.
Koshchei first appeared in Hellboy: Darkness Calls. Later he appeared in the How Koshchei Became Deathless, a back-up feature in Hellboy: The Wild Hunt. In late 2015, Mike Mignola announced he was working on a Koshchei the Deathless miniseries.