Create an eerie and mystical depiction of Koschei the Deathless, inspired by Slavic folklore. Imagine him as a tall, gaunt figure with an almost skeletal appearance, his skin pale and thin, revealing a ghostly presence. He should wear ancient, tattered royal robes with intricate Slavic patterns, symbolizing his undying, cursed nature. His eyes are cold and piercing, exuding both wisdom and cruelty, and he carries a long, twisted staff. The background should feel dark and mystical, with foggy forests or ruins, giving a haunting and enchanted atmosphere. Include elements like subtle glowing runes or ethereal lights that hint at dark magic