A brukołak is one of the many strange monsters found in the Dank Wilderness.
In the German translation of "The Edge of the World", there are creatures translated as "Wurdulaken" (plural). That same line in the English translation uses the term "were-brawls", and the Russian term, "wurdałak" may or may not be used in the Polish originals, but is listed on the Polish wiki as a variant of brukolak.
The "brucolak" is a species of Greek vampire. There is nothing particular regarding its description and appearance, except that it is the son of an excommunicated or most likely the excommunicated himself. It must be killed chopping its head and burning it on a wood fire. Then the dust must be thrown into the wind.
There is also a chance that the brukolak is a misspelling of the "vrukolak" or "Vukodlak", another species of Dalmatian vampire.