The koshchey is created through Alzur's Double Cross, a vile ritual known to some sorcerers. It is a member of that infamous group of gigantic beasts that can cause destruction on a massive scale while killing droves of people. Neither a crab nor a spider, it is tough as nails. Even a witcher might find the soft meat inside hard to get at.
- "What, in your opinion, is the best way to describe a koshchey?
- Death. I'd call it death."
Developer CD Projekt's characterization of the koshchey taken from the monsterbook, which was enclosed with the Collectors Edition of the computer game The Witcher for Poland, Hungary and the Czech Republic:
This monster owes its name to Russian folk tales, in which Koshchay the Immortal is a murderous sorcerer who renders himself immortal by concealing his life's essence inside an egg. He does eventually perish — when the egg itself is shattered. In Sapkowski's short story A Road with No Return, the koshchey is a nearly immortal monster, or at least one that proves extremely difficult to kill — all by virtue of having been created through a magic ritual known as Alzur's Double Cross, which also made the beast enormous in size. In the story, the protagonists barely manage to kill the creature: in the game, Geralt must face this challenge alone.
While working on the concept art and model of the koshchey, the artists looked to the animal world for inspiration, as they had in creating the kikimores. Sapkowski describes the beast as possessing the features of an arthropod — half-arachnid, half crustacean. It moves surprisingly fast for its size and sudden strikes of its forelegs would surely instill terror in anyone suffering from arachnophobia.