Koschei the Deathless (or 'the Immortal') is a figure in Slavic (Russian, Ukrainian, Polish) folklore. He is an extremely powerful sorcerer, sometimes referred to as a 'Tsar,' or king. His power seems to derive from two sources: one is his talented talking horse, which steed he received for serving the old witch Baba Yaga in her stables; the other is his detachable soul. This soul is described as being in a needle, inside an egg, inside a duck, inside a hare, inside an iron box buried underneath a green oak tree on a small island in the middle of a sea on the other side of the world. While his soul remains hidden, he cannot be killed and is physically invulnerable. One way to restrict his power seems to have been to imprison him; at the beginning of one tale the hero finds the emaciated Koschei in a secret room, bound with twelve chains and given neither food nor drink. The compassionate hero gives the sorcerer three barrels of water, after which Koschei recovers his strength, bursts his chains, and promptly abducts the hero's wife. It is only with the help of animal companions (including a superior horse that the hero also gains from Baba Yaga's stable) that he is able to overcome Koschei and recover his wife. In other tales the hero finds the egg with the sorcerer's life, and either snaps the needle or smashes the egg on Koschei's forehead, causing 'the Deathless' to die. Although never explicitly described in the tales, Koschei's name may derive from the word for 'bones,' and he is usually depicted as bony, even skeletal, with a long rough beard. From this it would appear that although his powers give him immortality, he does not have eternal youth into the bargain.
Koschei the Deathless appears in James Branch Cabell's Biography of Manuel, but there he is a demiurgic creator who appears in various guises. Koschei is the villain in the Soyuzmultfilm animation of The Frog Princess, and also appears as a character in the Hellboy stories.