The first authoritative 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 most numerous and richest written records are of West Slavic paganism, particularly of Wendish and Polabian tribes, who were forcibly made Christian only at the end of the 12th century. The German missionaries and priests who criticized pagan religion left extensive records of old mythological systems they sought to overcome. However, they hardly restrained themselves from "pious lies", claiming pagan Slavs were idolatrous, blood-thirsty barbarians. As none of those missionaries learned any Slavic language, their records are confused and exaggerated.
Major works include a chronicle of Thietmar of Merseburg from the beginning of the 11th century, who described a temple in the city of Riedegost (Radegast) where the great deity Zuarasic (Svarožič) was worshipped. According to Thietmar, this was the most sacred place in the land of pagan Slavs, and Svarožič was their most important deity.
Another very valuable document is the Chronica Slavorum written in the late 12th century by Helmold, a German priest. He mentions 'the devil' Zerneboh (Chernobog), goddess Živa, god Porenut, some unnamed gods whose statues had multiple heads and, finally, the great god Svantevit, worshiped on the island of Rügen who, according to Helmold, was the most important of all (Western) Slavic deities.