Veles (Volos) is the chthonic Slavic god of the underworld and of cattle, called skotiy bog, 'cattle god', in the Primary Chronicle, where the Rus swear by Perun and Volos in the treaties with Byzantium (907-971). Comparison with Varuna and with the Baltic Velnias makes him a god of magic, poetry, wealth and oaths, guardian of the boundary between the living and the dead, whose violation he punishes by 'turning a man yellow' (probably jaundice). The storm-myth duel with Perun is a 20th-century reconstruction by Ivanov and Toporov, criticised from the 1980s and not accepted as a documented narrative.
Modern Practice
Veles is called by herders for the herd, by poets and diviners for words that carry, and at boundaries, crossings and oaths. The old offering is the last sheaf left standing in the field for his beard, with milk, beer and a coin at the edge of the land, and the practice is a bargain rather than a devotion: you say what you will give, and you give it.