Slavic folklore is very different from its Western counterpart. Let’s start with fact that its most recognizable and quintessential antagonist is an ugly and emaciated dude, with “unblinking reptilian eyes [and] a raven’s beak of a nose [that] juts out over a cavernous mouth.” This beauty’s name is Koschei the Deathless, and his daily pleasures include waging war and stealing women. Koschei is derived from the Slavic word for bone (kostka). And, as his moniker suggests, he’s pretty hard to kill, though, it doesn’t mean he’s actually immune to dying.
According to legend, Koschei’s mortality is contained in a needle that’s placed somewhere highly inconvenient and very far. But, finding and breaking it is the only way to destroy him. It’s hidden in an egg, inside a duck, inside a hare that's inside a chest; the chest is buried under the roots of an oak tree, which grows on the invisible island of Buyan at an undisclosed location in the middle of the ocean-sea. It’s best to think of Koschei’s mortality as being stored in a type of Russian nesting doll commonly referred to as matreshka.
Though it may initially seem weird, the concept actually makes a lot of sense. If you somehow manage to get to the island — which on its own is clearly really, really hard — you still have to find the right oak tree to dig up the chest. Once you do so, you will encounter a hare that will try to escape. You have to trap the hare to get to the duck, which will attempt to fly away. But, you need the duck to retrieve the egg, so that you can get to the needle and finally kill Koschei the Deathless; or, you can use the egg to control him.
Some of his supernatural abilities include shape shifting, wielding power over the elements, and casting certain spells. And, even though he is perfectly capable of flying (by turning into vapor or the wind) he frequently appears naked as he rides a “magic steed” across the Caucus Mountains while wrecking havoc. Controlling Koschei the Deathless, however, does not mean you get to access his abilities. On the contrary, the longer you have the egg that contains his mortality, the less powerful he ultimately becomes.
Koschei the Deathless is not a monster or the devil. If anything, the guy's a walking contradiction. He was once a powerful sorcerer who, in his quest for eternal life, cast an unsuccessful spell that ultimately turned his mortality into an object, and weaved his being into the fabric of the world. As a result, he became a destructive force of nature hell-bent on following his most basic primal instincts. Still, despite all of his powerful magic and awesome abilities, he has his vulnerabilities, the main ones being the egg and the needle, and his relationship to them.
Think of it like this: The matreshka-style measures protecting Koschei's mortality are, in this case, symbolic representations of the world. The ocean-sea is water, the basin for life; the island is the soil that nurtures it; the oak tree is life itself; the animals are subsistence for the living; and the chest contains natural resources, the most precious one being the egg. The egg is an emblem of immortality, and the needle within is a representation of death. Together, they’re why the matreshka exists; they are what give it purpose and meaning.
But, where people meddle, trouble brews. Simply put, you must unravel the thread that holds the world together to kill Koschei the Deathless. So, it is really worth it?
Note* Illustration of Koschei the Deathless by Viktor Vasnetsov (1926 – 1927), Illustration of Koschei the Deathless (1901) by Ivan Bilibin | Both images are in the public domain.