A "black god" of misfortune mentioned in exactly one medieval source, a Christian chronicler's brief and moralizing account of a feast toast; the paired "white god" Belobog appears only in much later sources, and most of what is said about Chernobog today goes beyond the evidence.
Everything known about Chernobog comes from one passage in Helmold of Bosau's Chronica Slavorum (Book I, chapter 52, written c. 1168), describing Slavs at a shared feast passing a cup and speaking words invoking, as Helmold reports it, a good god and an evil one, "which in their language they call Zcerneboch, that is, the black god." Helmold states plainly that the Slavs believed good fortune came from the good god and misfortune was dispensed by the black one, and that they preferred to curse the evil god than to praise the good one. No other medieval writer, chronicle, or inscription mentions Chernobog.
What this site will not add
A popularly repeated "White God," Belobog, presented as Chernobog's opposite number, appears nowhere in Helmold or any other medieval source; the name is first attested only in 18th–19th-century antiquarian and Romantic writing, and modern Rodnovery (Slavic neopagan) material has built an elaborate dualistic cosmology around the pair that has no basis in the medieval record. Because Helmold is a single, late, foreign-authored, and explicitly moralizing witness, some historians have also asked whether his account reflects genuine Slavic dualism or a Christian writer's own framework of good and evil projected onto an unfamiliar toast. This site presents Chernobog only as far as Helmold's own words support, and no further.
Source Notes
These records lack edition and translator details. Their wording has not been verified as a direct quotation.
Among the various forms of their idols [...] the Slavs also have a strange delusion: at feasts and carousals they pass around a bowl over which they utter words, I will not say of consecration but of execration, in the name of the gods, of the good one as well as the bad one, professing that all good fortune is arranged by the good god, adversity by the bad one; hence also in their language they call the bad god Zcerneboch, that is, the black god.
Worship & Cult
How Chernobog was venerated in ancient times
Worship Practices
The only recorded practice is a feast-toast rite described by Helmold, in which a shared cup is passed while words are spoken invoking both a good god and Chernobog; no temple, idol, or dedicated festival is recorded anywhere else for him.
