These rare spawning ships would always travel within the fog, the audible clues one is near would be a mournful ringing of their ships bell and the creek of tormented wood on the waves. Perhaps a dull blue glow would be barely visible through the fog.
These craft would be swift in the water and sailors would normally only catch a glimpse of it before it sailed past. They would be non-aggressive to begin with so sailors could still use the fog as cover from other players.
You would have to use your lantern at all times if you managed to board this ghostly craft otherwise you'd fall straight through the hull as if it wasn't there.
on the lowest deck you'd find 3 items an ethereal chest, skull or crate of souls. The moment you picked one up and dropped your lantern you'd fall through the hull into the water.
They'd be a few seconds before an ungodly wail was heard across the waves and the ghost ship would materialise into the world bent on retrieving it's stolen cargo. You'd have the choice to stay and fight in the hope to retrieve the other two rewards or outpace the ship to the edges of the fog.
Both escaping and staying to fight have their risks. You'd have to be at full sail to stay ahead of the ghost ship with no idea if you were about to smash against the rocks or beach on an island. That moments delay could give the ghost ship the time it needed to pull along side you and send you to your grave,
Staying and fighting would also be risky. You'd be fighting against a ship unaffected by the fog whilst you struggle to see. The ghost ship wouldn't have cursed cannon balls but it could have 5 or 6 cannons along each side.
As per skeleton crews the ghostly crews can do everything but bail and would respawn until the boat sinks. Perhaps whilst fighting the craft the otherworldly influence would affect your navigational equipment sending your compass crazy as if you were in a storm making navigating the fog even more treacherous.
