Chernobog
Chernobog (Proto-Slavic *čĭrnŭ 'black' and *bogŭ "god"), also spelled as Chernabog, Czernobog, Chornoboh, Crnobog and Tchernobog is a Slavic deity, whose name means black god, about whom much has been speculated but little can be said definitively. The only historical sources, which are Christian ones, interpret him as a dark, accursed god, but it is questionable how important or malicious he was really considered to be by ancient Slavs. The name is attested only among West Slavic tribes of the 12th century, hence it is speculated that he was not a very important or very old deity. Older scholarship assumed him to be the counterpart of Belobog.
Sources
One historic source on Slavic mythology mentioning this god is the 12th-century Chronica Slavorum, a work written by German priest Helmold which describes customs and beliefs of several Wendish and Polabian tribes who were still pagans. Helmold wrote that:
Est autem Slavorum mirabilis error; nam in conviviis et compotacionibus suis pateram circumferunt, in quam conferunt, non dicam consecracionis, sed execracionis verba sub nomine deorum, boni scilicet atque mali, omnem prosperam fortunam a bono deo, adversam a malo dirigi profitentes. Unde etiam malum deum lingua sua Diabol sive Zcerneboch, id est nigrum deum, appellant.