In Slavic folklore, Koschei (Russian: Коще́й, tr.Koshchey, IPA: [kɐˈɕːej], also Kashchei or Kashchey; Ukrainian: Кощій, Koshchiy; Polish: Kościej; Czech: Kostěj) is an archetypal male antagonist, described mainly as abducting the hero's wife. None of the existing tales actually describes his appearance, though in book illustrations, cartoons and cinema he has been most frequently represented as a very old and ugly-looking man. Koschei is also known as Koschei the Immortal or Koschei the Deathless (Russian: Коще́й Бессме́ртный, Ukrainian: Чахлик невмирущий or Кащик невмирущий, Czech: Kostěj nesmrtelný), as well as TsarKoschei. As is usual in transliterations, there are numerous other spellings, such as Koshchei, Kashchej and Kaschei. The spelling in Russian and other Slavic languages (like Polish "Kościej" or Czech "Kostěj") suggests that his name may be derived from the word kost' (rus. кость, pol. kość) meaning 'bone', implying a skeletal appearance.
Koschei cannot be killed by conventional means targeting his body. His soul (or death) is hidden separate from his body inside a needle, which is in an egg, which is in a duck, which is in a hare, which is in an ironchest (sometimes the chest is crystal and/or gold), which is buried under a greenoaktree, which is on the island of Buyan in the ocean. As long as his soul is safe, he cannot die. If the chest is dug up and opened, the hare will bolt away; if it is killed, the duck will emerge and try to fly off. Anyone possessing the egg has Koschei in their power. He begins to weaken, becomes sick, and immediately loses the use of his magic. If the egg is tossed about, he likewise is flung around against his will. If the egg or needle is broken (in some tales, this must be done by specifically breaking it against Koschei's forehead), Koschei will die.
In Alexander Veltman's Koshchei bessmertny: Bylina starogo vremeni (Koshchei the immortal: A bylina of old times, 1833), a parody of historical adventure novels, the hero, Iva Olelkovich, imagines that his bride has been captured by Koschei.
Mercedes Lackey's novel of Stravinsky's Firebird features Katschei as the main villain, retelling the classic tale for a modern audience. Also, in her 500 Kingdoms series, the Katschei is referenced in the novels The Fairy Godmother and "Fortune's Fool."
Koschei appears as an antagonist to the heroic demon Hellboy in the 2007 comic bookHellboy: Darkness Calls. The Baba Yaga will grant him death only through Hellboy's destruction, but in Hellboy, Koschei's soul is hidden in an egg, inside a duck, inside a hare, inside a goat. Vasilisa Prekrasnaya also appears and helps Hellboy. Koschei's origin story is later revealed in (as yet uncollected) backup stories to single issues of Hellboy: The Wild Hunt.
A reference to Koschei is found in the role-playing game book The Plains of Howling Darkness of the Fabled Lands series. An immortal tyrant named Kaschuf the Deathless keeps a steppe village under his yoke until the player releases his soul from a locket kept hidden on an island in another book in the series, Over the Blood-Dark Sea.
H. Beam Piper named an industrial planet Koshchei in his novel Junkyard Planet aka Cosmic Computer about the search for a computer called Merlin that was so advanced it could predict the future.
Koschei was the name given to an early incarnation, and also an alternate version, of the Master in the Doctor Who range of novels.