Pirates of Nether Port
The first thing you might realize is that by the time the nineteenth century rolled around, there were no pirates left in the world as we traditionally think of them. Pirates that flew under the skull and cross bones had mostly existed at least hundred years before the Blackburns ever came on the stage. How it was that these buccaneers still walked this earth is a mystery that would come to light in time. For as sure as day, there were swashbuckling sailors, and they were a force to be reckoned with, to be sure. It was only after coming upon the strange island of Avalon that Mortimer had his first run-in with these dire and murderous men. And they had powers that traditionally are ascribed to only sidhe. This gave Mortimer quite a start, and they were a bit more of a challenge than a capable wizard was accustomed to having with such coarse and inhuman humans. But they had more than one magical card up their sleeves, having lived and made berth for many years at the place they called Netherland, or Nether Port.
How they predictably remained able to reach these digs is a story for another day, only know that in the grand scheme of things and over millenia, they were the only inhabitants that had until then mastered access to this mysterious place; most after finding it could never find it again.