In Slavic folklore, Koschei (Russian: Коще́й, tr. Koshchey; IPA: [kɐˈɕːej], also Kashchei or Kashchey; Ukrainian: Костій, Kostiy; 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: Кості́й Безду́шний or Кощі́й Безсме́ртний, Czech: Kostěj nesmrtelný), as well as Tsar Koschei. 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 iron chest (sometimes the chest is crystal and/or gold), which is buried under a green oak tree, 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 needle is broken, Koschei will die.
In folk tales
Koschei also appears in Russian versions of the story "The Frog Princess".
In popular culture
- In James Branch Cabell's Jurgen, A Comedy of Justice, Koshchei the Deathless appears as the most supreme being who made things as they are and is therefore universally unappreciated before Jurgen's kind words are spoken.
- In Vasilisa Prekrasnaya (Vasilisa the Beautiful), a Russian cartoon based on the Russian fairy tale.
- A villain in Igor Stravinsky's Firebird.
- 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.
- Nikolai Rimsky-Korsakov wrote an opera involving Koschei, titled Кащей бессмертный, or Kashchey the Deathless.
- In the Soviet animated film Beloved Beauty (1958).
- 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 a slave to Baba Yaga in the Hellboy comic book series, his soul hidden in an egg, inside a duck, inside a hare, inside a goat. First appearing in Hellboy: Darkness Calls, Baba Yaga sends Koschei to kill Hellboy in return for his freedom. Vasilisa Prekrasnaya also appears and helps Hellboy. Koschei's origin story is later revealed in backup stories to single issues of Hellboy: The Wild Hunt. The story is also collected in Hellboy: Weird Tales.
- In The Sandman: Fables and Reflections, Koschei's emerald heart (or a piece of green glass passed off as such) passes into the possession of a Romani trader, then a werewolf, then Baba Yaga, and is last seen as a medallion worn by Dream's successor, Daniel Hall.
- In Monday Begins on Saturday by Arkady and Boris Strugatsky, as one of the creatures held in the NIIChaVo institute.
- Koschei appears as a character in the MMORPG RuneScape, under the name "Koschei the Deathless".
- Koschei appears as a character in John C. Wright's "War of the Dreaming" novels. He offers to save the hero's wife, if the hero will agree to take the life of a stranger.
- In the Doctor Who expanded universe novel The Dark Path, the Doctor's arch nemesis, The Master is using the name "Koschei".
- Catherynne Valente's novel Deathless is a retelling of the Koschei story set against a backdrop of 20th-century Russian history.
- In the video game series The Incredible Adventures of Van Helsing, the Death of Koschei is a key plot item in the second game. In the third game, re-curring supporting character Prisoner Seven is revealed to be Koschei the Deathless, and becomes the main antagonist.
- In the Webcomic PS238 by Aaron Williams, the child hero 84 is currently trapped in Koschei's egg, trying to find the "eye", and in doing so, will become his new Champion of Earth to battle from now on.
- In the videogame Shadowrun: Hong Kong, the supporting character Racter has a drone named 'Koschei'. If questioned, Racter will reveal that he named the drone thusly because he has full backups of every aspect of it, allowing him to rebuild the drone no matter what might happen to it.
- In the computer game Dominions 4: Thrones of Ascension, Koschei appears as a hero character for the faction of Bogarus, a faction inspired by medieval Russia and Slavic mythology.
- The Death of Koschei the Deathless
- Baš Čelik, arguably the same figure in Serbian folklore.
- Author interview: 
|Wikimedia Commons has media related to Koschei.|
- Sir James George Frazer (1854–1941). The Golden Bough.
- The Golden Bough by Sir James George Frazer (Project Gutenberg)
• Jurgen: A Comedy of Justice by James Branch Cabell which features the power of "Koshchei the Deathless, who made things as they are".
- The Death of Koshchei the Deathless from Andrew Lang's Red Fairy Book
- Old Russia Entry on "Koshchey" also contains a link to a folk story proper.
-  Referencing Episode.