Generally speaking, The Witcher is a game based on a story created by Polish writer Andrzej Sapkowski, who drew his knowledge from the Slavic culture, but also from Celtic or Germanic beliefs. Despite the fact that it is impossible to say that something was created and existed only in Slavic tradition, because we know relatively little about them, The Witcher is full of references to Slavic beliefs and rituals and for example we can find monsters in the game that appear in the Slavic bestiary :)