“Now there was there nigh unto the mountains a great herd of swine feeding.”
A large herd of pigs was feeding on the nearby hillside — the presence of pigs confirms the Gentile setting: pigs are unclean animals that no observant Jewish household would keep. The large herd (about two thousand, according to verse 13) represents a significant economic investment for the herdsmen and their owners. The irony is available but not pressed by Mark: the unclean spirits in an unclean man are about to go into unclean animals. The three-fold uncleanness of the scene (Gentile territory, demon possession, pigs) communicates that the kingdom's reach extends to the most ritually impure context imaginable.