Complex sentence: Perun, being the god of thunder and sky, was worshipped in high places, whilst the places reserved for Veles, the god of underworld and cattle, were in lowlands near rivers or springs.
Split into simpler sentences: Perun, being the god of thunder and sky, was worshipped in high places. On the other hand, the places reserved for Veles, the god of underworld and cattle, were in lowlands near rivers or springs.