Chernobog is attested once, in Helmold of Bosau's 12th-century Chronica Slavorum: at their feasts the Slavs pass a bowl and call on the good god and the bad god, naming the bad one Diabol or Zcerneboch, 'the black god', who arranges adverse fortune. Every later source is derivative - Johan Lindner (c. 1530), Thomas Kantzow's Pomeranian chronicle (1538), Sebastian Münster's Cosmographia (1550), the Lusatian compilations - and the claim that he received human sacrifice comes from Kantzow, a hostile 16th-century chronicler, not from a pagan source. Scholars disagree whether he was a real Polabian god of misfortune or a learned gloss on the Christian devil.
Modern Practice
The older practice is a bowl poured out at the feast in his name so that he stays outside the door, not a cult. Modern work with him treats him as the shadow that must be acknowledged and dismissed: a portion set aside, a name spoken once, and no invitation.