Perun is a Slavic God of sky, thunder and rain. Svarog was also considered as God of sky, however, Perun ruled over physical, atmospheric realm, while Svarog ruled over Prav - Realm of Gods and spirits of dead.
Myths & Legends
Perun was tied to justice and order. He could punish evil people by closing the gates of Iriy (place in realm of Prav dedicated to virtuous people), or by striking them down with lightning bolts. The oak tree and ''perunika'' (iris germanica) flower were also his symbol.
As the God who had the strongest cult, next to Dažbog and Svarog, Perun was considered the most powerful God in Slavic folklore. Historians, not even today know if he was chief God of all Slavs. Because of his role of bringer of thunder and rain, all Slavs, no matter of tribe, paid him a tribute in form of food or ''dodole dancing'' to battle the drought, especially during Summer season.
He is represented as muscular, bearded warrior with a mastery of axes He rode burning chariot pulled by fiery-mane horses who breathed fire and sounds of thunder were actually sounds of His chariots.
Battle between Perun and Veles
Perun's main weapon was an axe, decorated with thunder marks, and golden apples. Whenever Perun tossed a golden apple into the air, they would transform into thunder and lightning. He used both against Veles - His arch-enemy.
There is story that Veles, in form of a horned serpent would slither his way to top of World Tree, causing Yav - human world to run dry. However, Perun would battle him with his axe and eventually kill the serpent, throwing him down to the ground and announcing victory by bringing rain and thunder. But Veles never dies and the cycle of battle, death and rebirth is repeating every time before rain happens.
Though Perun and Veles are pretty much neutral in mentality, their battle is still considered as battle between order and chaos.
Perun in Christianity
During Christianization of Slavs, there was religious phenomenon called ''dvoverje'' (having two faiths at same time). Slavs didn't want to abandon their Gods so easily. However, what Christian priests discovered is that days of celebration of their Gods is close to days of Christian Saints. And so people exchanged Perun with St. Elijah (who was also celebrated at 20th of July). St. Elijah among Southern Slavs was also called The Thunderer and got exactly same role as Perun - causing thunder and rain and riding burning chariots across the sky.