Perun was the thunder god of pre-Christian Russia. His name means approximately "the striker" (per- meaning to strike and -un being a suffix that denotes agency of an action). Perun was identified at least to some extent with the Greek god Zeus, the two being occasionally interchangeable in some older Slavic translations of Greek classics. Perun is also mentioned in connection with other Slavic deities, such as Chors, Volos, Vila, Rod, and Rozanica, although the relationship between the various belief systems here is somewhat uncertain. It is pretty probable, though, that other Slavic peoples worshipped either Perun or some Perun variant.
The idol of Perun that we know most about today belonged to Prince Vladimir of Kiev. It had a silver head and a golden beard and was set on the hill in front of his palace in 980 C.E. For the ruler of all Russia to set Perun's idol not only in front of, but also above his palace should give some indication of the importance of Perun, who was generally considered to be the primary deity of Russian paganism. In fact, when pagan Russia swore by anything, it swore by Perun. In 945, the Russian state was in the process of concluding a treaty with Byzantium. When all the wheeling and dealing had taken place and the treaty was hammered out to the satisfaction of all concerned, the Russians, led by Prince Igor, went to the hill where Perun's statue was (whether Perun was always worshipped on hills or not is uncertain, although I'd imagine that was probably the case, given his status as a sky/thunder god). While the Christian Russians swore in the local church of St. Elias to honor the treaty, the pagans swore in front of Perun that the following things should happen to them should they break the treaty:
"Let them never recieve aid either from God or from Perun; let them never have protection from their shields; let them be destroyed by their own swords, arrows, and other weapons; and let them be slaves throughout all time to come."
Perun fell from the favor of Russia when in 988 Prince Vladimir underwent baptism to Christianity. He went on an idol-wrecking tear after that, and Perun was not spared. They tied the statue to a horse and dragged it to the Dnieper river, where it was beaten with rods. Yes, they beat a statue with rods, apparently to disgrace it for having decieved them so long. The pagans reportedly wept as the statue was thrown into the river since they hadn't undergone the same conversion as their ruler. The statue was ordered to be floated down the river "past the rapids," after which point Vladimir said he didn't care what happened to it. When the statue was past the rapids, it immediately came to a stop on the sandy shore, which from then on was referred to as Perunya Ren or Perun's sands. In Novgorod, where they also had an image of Perun on a hill, the same thing happened in 989, and happened via virtually the same method as in Kiev: the statue was dragged with ropes through the mud while being beaten with sticks and reportedly crying out in pain. This statue was cast into the Volkhov river, where the next day a man who dwelled on the banks of a tributary, the Pibda, saw it floating toward the shore and pushed it back into the river, saying, "Now, Perunisce (a contemptuous diminutive), you have had enough to eat and to drink; be off with you!"
The story of Perun doesn't stop there, though. Although it was about a century before the people of Russia actually stopped worshipping Perun, his worship found ways to persist anyway. St. Elias, the saint to whom the Christians swore while the pagans swore to Perun, doubled for the god in other ways after the rise of Christianity in Russia. On St. Elias' day, July 20, it sometimes occured in Russia that enormous feasts would be held at which livestock would be sacrificed after their consecration in the local church. In the Rhodope mountains, this feast was even held on the summit of a mountain. Additionally, the Serbians refer to St. Elias as either Gromovnik or Gromovit, which means the thunderer, and pray to him for good harvests. The source from which I'm getting this information even speaks about vicarious Perun worship through St. Elias in the present tense, albeit present tense circa World War I. St. Elias, then, probably took a similar cultural niche to the one his pagan predecessor Perun did, as so many other Christian customs did throughout Europe after the abandonment of paganism.
Source: Gray, Louis Herbert. The Mythology of All Races, Volume III. Boston, 1918.