Veles is the great counterweight to Perun in the Slavic cosmos — lord of the underworld, of wealth and cattle, of magic, shapeshifting, and the arts. Where Perun rules the sky, Veles rules the roots of the World Tree. Where Perun governs warriors and kings, Veles governs wizards, musicians, and merchants. He is not evil — he is chthonic, underworld, the necessary darkness that makes the light meaningful. His conflict with Perun is not a war between good and evil but between the two poles that must remain in tension for the world to function.
When Christianity came to Slavic lands — in waves from the 9th to 14th centuries — it did not erase the old gods but reinterpreted them. Perun, sky god and ruler of warriors, mapped easily onto the Christian God and his saints (particularly Elijah, whose fiery chariot provided a thunder-myth substitute). Veles, lord of the underworld and master of shapeshifting and magic, mapped onto the Devil.
The association stuck so thoroughly that the Slavic word for "devil" in several languages derives from Veles's name. His serpentine form, his underworld domain, his association with magic and hidden knowledge — all were reread through the lens of Christian demonology. But folk belief retained traces of his original character: the harvest festival customs of leaving grain for Veles, the reverence for cattle given to his protection, and the persistence of his name in place names and personal names across Slavic lands.
Veles is not the devil. He is what every culture eventually turns its old chthonic gods into when a new sky religion arrives — the necessary darkness rebranded as evil, the underworld wisdom repackaged as forbidden knowledge.
— Lotte Motz, The Faces of the Goddess