The Witcher Bestiary - Koshchey
Koshchey Occurrence: A mythical creature born of magic; only a mighty mage who knows Alzur's work can summon a koshchey.

Immunity: Resistant to most effects.

Susceptibility: Susceptible to silver.

Tactics: This relentless killing machine has no need for tactics.

Alchemy: Koshchey heart.

What, in your opinion, is the best way to describe a koshchey?

Death. I'd call it death.