According to the Book:
Another legend that causes chills and sleepless nights in the ports and villages of the Principalities is the story of Prince Moren and his Ship of Bones. The legend claims that the evil Prince Moren was cursed, and now he sails the Lhazaar Sea in his skeletal ship with a crew of undead pirates in search of living sailors on which to feed. It is said that on nights when two or more moons are full, the Ship of Bones slides silently over the waves to attack hapless vessels unlucky enough to cross its path.