It began as a merchant ship known as The Stallion but, shortly after its third voyage, it was taken by pirates of Phobos and renamed The Corsair. It ran aground while trying to escape several of ships of the King’s Fleet.
They say that it lay on the bottom, its sails still catching the breeze, not quite sunk, not quite afloat, for three weeks until it finally shifted out of the water. It remained abandoned for a full month before the Pirate King Caven found it.
He re-christened it “The Relentless” and bestowed on it the ability to “never be sunk the same way twice”. It can, therefore, never run aground again, always finding safe paths through reefs and shoals.