Spelljammer-ish campaign idea
So, the setup: the players are at a tavern in a worldlet resembling a sphere of ocean water floating in space with tropical islands dotted across its surface, or bands of water floating in space with the islands floating on them. They hear a commotion outside at the docks, and someone bursts in and says that a famous pirate ship has returned. At the docks, the wrecked form of a ship flying a ragged pirate flag drifts in the distance, slowly limping towards the dock. The dockmaster sends out a tug to pull it over. The ship appears derelict and abandoned, but as the tug workers begin to affix ropes to it, a heavily injured man crawls up from bellowdecks. As the wreck is pulled back, those assembled at the dock can hear the man raving about how he's the only one left and that someone or something is chasing him and is almost there. As the man is carried back ashore by the tug workers, the island's bell tower rings three times, a signal that its scrying team has detected hostile...