Player has to go on a quest to visit the high holy spot of their Good deity. In the course of the trip during a sea voyage they are beset upon by a ship of the undead. Given that the ship's complement is unbeatable, they disable it at the helm or rudder and leave it to crash on the reefs. Continue on with visiting the nice deity and when they finally arrive after whatever other challenges you choose to put in their path (I used lots of spirit things, including ghosts of monsters they had recently defeated and of dead friends) they are told by the deity/priests that tho their actions were commendable they must finish their business with the undead ship. It seems that since it has grounded on the reefs whatever eldritch energies went into powering it are now slowly puncturing a hole to the plane of the undead. In my version, the ship is half filled with water and as the waves pass over/through it the battles got very messy. In the end it was a second wheel below decks that was the focus of the power, manned by a gent cursed for killing women and children on a particular ship he plundered.