Ghost ships are a common trope in fantasy and horror fiction. (And real life as well—the Wikipedia page on them is riveting.) But bone ships are something else entirely—no mere derelict or phantom or even the classic ship crewed by lacedons and draugrs. Instead of being crewed by undead, the bone ship is itself the undead creature, a thing of tissue and muscle and sinew and negative energy and, yes, lots and lots of bones. And if that doesn't get you excited enough, it’s a CR 18 creature. It trails a wake of blood and sends out ghostly boarders. Even though it is a singular entity, it also possesses a hive mind of souls (that, naturally, can drive others insane).
But here are the things that are going to work for you or not: It can form four cannons that fire bone shards. And it can combine those cannons into a single cannon of spectral energy. It also heals itself by crawling along the bottom of the ocean scavenging the corpses of sea creatures, and may (the flavor text says yes, but the stat block doesn't list a land speed) be able to crawl along the shore to hunt prey.
That’s an image that some GMs are going to love. A living ship that scuttles along the ocean floor absorbing whale bones and then rises to the surface to spontaneously generate cannons of bone and spectral energy is a notion that, if it doesn’t work for you, doesn’t work for you…but if it does work for you, it reeeally works for you.
As a CR 18 monster, a bone ship can stalk your PCs for months of near-misses, until in the end it is one of the final baddies your players face in a campaign. And since cannons are involved, this is an undead creature that easily plays outside the usual late medieval/early Renaissance fantasy milieu. In fact, you might find some more modern (and not) ideas in the adventure seeds below.
Also as you might expect, since these ships are so powerful, they don’t have your run-of-the-mill creation stories either. Gods make these ships. Disasters make these ships—not your average shipwrecks, mind you, but all-hands-down-with-the-ship-and-all-the-hands-were-evil-cultists disasters. And evil rituals make these ships—again, not the kind that involve sacrifices, but the kind that involve abattoirs. Bone ships are bad news…and even when their creation appears accidental some dread purpose, compulsion, or divine hand almost inevitably reveals itself.
Norse legends tell of Nalfgar, Hel’s ship made entirely of the fingernails and toenails of the dead, which Loki or Hrym will ride to Ragnarök. Adventurers on their way down the trunk of the World Tree to battle Níðhöggr follow a root into a strange harbor. There they discover the myths have both taken a bit of poetic license and understated the threat. Hel’s ships are not made of nails but bone and muscle…and she has a fleet of them.
The Templars of Jianvia are held in suspicion—quite rightly—of being too enamored with the clink of gold and the ledgers of finance than with containing the div hordes and corrupted night couatls of the Flamelands. That said, their expertise in the handling and disposal of dangerous artifacts is second to none. So it is a double tragedy when the galleon carrying both the head of the Jianvians and the Dread Hand of the Butcher is lost to a squall. Adventurers soon discover that not only did servants of the Great Old Ones call the squall, but the lost galleon already sails the seas once more. In its madness the ship is still trying to deliver the Dread Hand, even as it attacks or drives insane all who come into contact with it.
Few hatreds are as enduring as the hatred of the slaver for the people who have thrown off the yoke of oppression. In the bloodiest days of the American Civil War, a convoy running guns and slaves to Charleston is set upon by the Union’s Colored Silver Dragon Corps. The Union corpsman corral all the ships but one—the convoy’s flagship, whose white supremacist commodore slaughters his own sailors and scuttles his own ship rather see freed slaves set foot upon his deck. In fact, the man’s hatred and the suffering of the human cargo that went down with the ship combine to reanimate the vessel as a bone ship. Now adventurers must destroy the undead monstrosity as it hunts Union ships and merchants up and down the coast.
—Pathfinder Bestiary 5 44–45