They have been searching for it for a very, very long time.
Other civilizations have legends of the same- elusive vengeance-takers, ghost ships, sometimes even constant enemies. They lead long chases over fissure-covered icy worlds, through ragged nebulae, and occasionally even into oceans of gas and liquid surrounding rocky cores. They may exact a toll, sing songs of the dead, or simply fire upon all who see them.
This one is not like its compatriots. All you may see of it is the occasional flash of a fin as it accelerates past, and all you may hear is a quiet laughter piped in through your comms systems.
The chase goes on.
