Koschei is a deathless villain of East Slavic tradition who has hidden his death on the point of a needle, the needle inside an egg, the egg inside a duck, the duck inside a hare — layer within layer. Unless that death is found and broken, he can never die.
As a being who has detached his death and hidden it outside his own body, he embodies an immortal dread that seals its one weakness within a core forever out of reach.