Hellhound known as the Beast is giant specter hound that terrorized people. It is the main antagonist of the act 1 of the first witcher game.
It appears in places inhabited by evil people to terrorize them and slaughter for their sins. It appears only at nights in charge of pack of barghests, lesser specter hounds. Those hounds are made from common dogs or their corpses. It has also power to trap its preys in a cricle of fire.
Reverend (Witcher) accusses witch Abigail for summoning the Beast but in fact he and his followers are responsible for this because they are all villains - murderers, liars, rapists, slavedrivers, etc. Witcher Geralt slays the Beast at the end of act 1.
"The hellhound, referred to also as the Beast, is a creature of the underworld, a specter that assumes the form of a terrifying hound and tirelessly stalks its victims once it finds their trail. Peasants believe that the hellhound embodies the vengeance of the gods, who send the Beast to punish humans for their wickedness."