There are no known written accounts of Slavic mythology predating the fragmentation of the Proto-Slavic people into Western, Eastern, and Southern Slavs, with the possible exception of a short note in Herodotus’ Histories, mentioning a tribe of Neuri in the far north, whose men, Herodotus claims, transform themselves into wolves for several days each year. Some researchers have interpreted this through the Slavic folk belief in werewolves, whilst others believe that Herodotus actually referred to ancient Slavic carnival festivals, when groups of young men roamed the villages in masks, sometimes referred to as vucari (wolf-humans). The identification of "Neuri" with Proto-Slavs remains controversial, however.
The first definitive reference to the Slavs and their mythology in written history was made by the 6th century Byzantine historian Procopius, whose Bellum Gothicum described the beliefs of a South Slavic tribe that crossed the Danube heading south in just two days. According to Procopius, these Slavs worshipped a single deity, who crafted lightning and thunder. Though not named explicitly, it can be deduced this is a reference to the deity known as Perun in later historic sources, as in many Slavic languages today (Polish 'piorun' for example). Perun simply means "thunder" or "lightning bolt". He also mentions the belief in various demons and nymphs (i.e. vilas), but does not mention any other names.
The Slavic Primary Chronicle is a major work with many valuable references to the pagan beliefs of Eastern Slavs. The chronicle treats the history of the early Eastern Slavic state. Even though the manuscript was compiled at the beginning of the 12th century, it contains references to and copies of older documents, and describes events predating the Baptism of Kiev. Two deities, Perun and Veles/Volos, are mentioned in the text of the early 10th century peace treaties between pagan rulers of East Slavs and Byzantine Emperors. Later, Nestor the Chronicler describes a state pantheon introduced by Prince Vladimir in Kiev in 980 CE. Vladimir's pantheon included Perun, Hors, Dažbog, Stribog, Simargl, and Mokosh. The Hypatian Codex of the Primary Chronicle also mentions Svarog, compared to Greek Hephaestus. Also very interesting are the passages in the East Slavic epic The Tale of Igor's Campaign referring to Veles, Dažbog, and Hors. The original epic has been dated to the end of the 12th century, although there are marginal disputes over the authenticity of this work.
The Indo-European custom of communal feasts was known as bratchina (from brat, "brother") in Kiev Rus, as slava ("glorification").
Statues of several Slavic deities were discovered in 1848, on the banks of the Zbruch river, a tall stone statue was found, with four faces under a single stone hat.
The remains of several Slavic shrines have been discovered. Some archeological excavations on the cape of Arkona on Rügen island have uncovered vestiges of a great temple and a city, identified with those described by Saxo. In Novgorod, at the ancient Peryn skete, archeologists discovered the remains of a pagan shrine likely dedicated to Perun. The shrine consisted of a wide circular platform centred around a statue. The platform was encircled by a trench with eight apses, which contain remains of sacrificial altars.
All these archeological remains have the multiplicity of aspects in common. Statues of gods with multiple faces and remains of shrines with multiple sacrificial altars confirm written reports of Christian missionaries about the Slavs worshipping polycephalic gods, and also indicate that ancient Slavic mythology apparently put great emphasis on worship of deities with more aspects than one.
Also quite important are remains of several pieces of pottery from 4th century Chernyakhov culture. Russian archeologist Boris Rybakov identified and interpreted symbols inscribed onto them as records of the ancient Slavic calendar.
It is commonly claimed that worshiping among Slavic people often happened in the woods rather than in shrines. Such woods were called "gaje" in the Proto-Slavic language (compare Polish gaj 'small wood, thicket, bush, grove'; see: sacred grove), and were sometimes encircled by a fence which created a sacred area, both a natural and social sphere. Sometimes they would be used as cemeteries as well.