Tchernobog was a (battle?) god of the Baltic Slavs, whose territory lay in what's now Northern Germany and part of Poland. They were direct neighbors of the Vikings, if you will, and had intense feuds and trade with them as well as their own alphabet, called Runitsa, which was forbidden when they were Christianised (hence the modern use of the Cyrillic alphabet). Bog means god.
Remarkably, they resisted Christianisation until they were systematically assimilated/oppressed/raped of their cultural identity by means of a settlement policy, not unlike Israel's settlements in the West Bank, in the 12th and 13th century. Systematically, Frisian and other settlers were transplanted to their area, which there is still evidence of in place names. Christian settlements were often built right next to the Wendish ones, and the place name would be prefixed with a "Frisian" or "German".
This means they were among the last Pagan cultures in Northern Europe or the Baltic to succumb to Christianisation. One reason was that they had an influential priest caste, which the neighboring Germanic pagans were lacking (in their case, kings or other important men also presided over offerings).
Their more well known god was Svantevit, a main temple to whom stood on Cape Arcona, Rugen.
The Baltic Slavs burned down Hedeby in 1066, which kinda officially ended the Viking Age. Damn. :-P
Around 1147, they had their own crusade dedicated to them (!). Christian commentators lamented at the time that the crusaders were more interested in plundering than spreading the faith. Not surprisingly, the crusade was a failure in everything but raiding, raping, and making slaves.
You kinda have to salute these people for being so tough, and sticking to their old ways against all odds. It was genocide, really. Their culture is de facto extinct today, although their descendants of course live. But there is no awareness of a Baltic Slav identity, unlike with the Sorbs or Danes or Frisians in Germany.
There is a statue of Svantevit on Arcona again, though:
The original was broken down by the missionary Absalon around 1168, with the help of Valdemar of Danmark: