Merfolk were not a warlike people. They actively stayed away from it if anything. There was a reason why they were secluded in Black Lake on Hogwarts lands. The open ocean was a dangerous place for an underwater civilization. After the destruction of Atlantis, the Merfolk had been trying to find a place for themselves. Their people scattered across the winds.. sea and spread across the globe. One particular sect made their way to the British Isles. 