Cultural background for Czernobog
Here is what I wrote for Czernobog’s cultural background (note that I also added Bielobog in it, for reasons obvious to the novel reader)
Czernobog (or Chernobog, the “black god”) and Bielobog (or Belebog, the “white god”) were Slavic deities worshipped by the Polabian Slavs, and a representation of the fundamental duality of the Slavic mythologies (some even considered them Western Slavic nicknames for the dualistic gods Perun and Veles). While they are some of the most widespread and well-known Slavic gods in popular culture, their existence is heavily debated. Indeed, there is no absolute mythological proof of their existence, only late records by foreign Christian scholars and suggestions of their existence in Western Slavic language and culture. The most common theories are that they are an invention of Christian monks, a misunderstanding of the worship of Perun and Veles, or minor local deities given much more importance by the Christians writings during the Christianization of the Polabian territories.
According to foreign Christian sources, from the 12th to the 16th centuries, Bielobog was a benevolent god of good luck, light and the day, and Czernobog a negative god of misfortune, night and darkness. While Bielobog was honored so that he would bless humans with good things, Czernobog was honored so that he would not bring harm to humans. It was said that Czernobog was worshipped through human sacrifices, only human death and human blood being able to appease him. Christian priests taught that Bielobog was actually the Christian God, while Czernobog was the devil or Satan – to the point that some consider Bielobog and Czernobog to have originally been naturals gods of light and darkness that the Christianization of the Slavs turned into figures of good and evil (the same way the other dual gods of Slavic mythology, Perun and Veles, were compared to God and Satan by the Christians).
Outside of those written records, other elements possibly attest the existence of the two gods. Two mountains were named after them (Czorneboh and Bieleboh) because they presumably hosted their cults, though it is considered that these names are 18th-century inventions. The Arabian medieval historical account “The Meadows of Gold” mentions an idol at the top of the Black Mountain, depicting an old man surrounded by ants, ravens and black-skinned people: it has been considered to be Czernobog. Several towns and places in Russia, Ukraine and the Czech Republic might have been named after the two gods. Several expressions and sayings were also linked to the deities: “May the black god smite you!” (Ukrainian), “To not see a white god” (Serbian), “To cry out a white god” (Bulgarian) or “I have no white god from this man” (Bulgarian, means that someone lacks good will). Finally, the folklore of White Russia mentions a supernatural figure named Bieloun, an old man with white clothes and a white beard, appearing only during the day and helping all the unhappy people he meets, be it guiding lost travelers or sharing the peasants’ labor.
Chernobog’s name was heavily popularized thanks to Disney’s adaptation of “Night on Bald Mountain” in the musical movie “Fantasia”. Depicting the witches’ Sabbath according to old Russian legends, the devil leading the infernal reunion was called “Chernabog”, a reference to Chernobog, though Walt Disney and Deems Taylor both explained that Chernabog was the same as Satan.
