Perun was an important Slavic god, who presided over the sky, thunder, lighting, storms, rain, law, war, fertility and oak trees.
Perun was the ruler of the living world (earth and the heavens) often described residing atop of the World Tree, taking the shape of an eagle, and is opposed to the god of Veles, who was the ruler Nav, the Slavic underworld which was located at the roots of the tree. He is also often described wielding either an axe or a hammer, and traveled through the sky in a chariot, controlling thunderstorms and lightings.
He was the son of Svarog and Lada, the god of fire and blacksmithing and the goddess of fertility and beauty, brother to Dažbog, the god of the sun, and husband to Mokosh (or Perperuna), the goddess of life, fertility and moisture, and father of Jarilo and Morana, god of vegetation, fertility and springtime and goddess of cold, frost, winter and death.
Veles would often appear from the underworld, cause chaos on the Earth, and tries to steal either Perun's cattle, children or wife. Perun would start chasing Veles and cast lighting at him, which leads to the underworld-god to either transform into animals or hide behind trees and houses. Perun would eventually either kill Veles or chase him back into the underworld, and establish order in the world.
Perun, like many other pagan gods, was demonised with the arrival of Christianity. One notable example of this was Grand prince Vladimir the Great, who was once a follower of the god, but after converting to Christianity, started to destroy pagan monuments. The statues of Perun were notably dethroned, beaten with sticks and eventually cast into the river. They were also not allowed to land on shore.