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.