They could see the ship drifting towards their docks. It was derelict and they could smell the disease as it floated closer. The townspeople were able to make out the teeming mass of ravagers on the deck and in the rigging.
They knew that each man on the ship was a herald of death, armed as they were with salted steel. Beyond that though, each was covered with eruptions from one foul disease or other, and that was the real danger.
The townspeople had heard stories of places that had repulsed the raiders, only to succumb to their blasphemous plagues.