Coelomia: island of secrets. For the Sea of O’Sr
From a ship, Coelomia looks like a low, rocky sandbar or reef, or possibly the mouth of a crater, with a shallow warm water lagoon in the middle.
It could easily be mistaken for the South Pacific island of Tabuearan. Outside the ring of visible land there’s a steeply-shelving, fairly smooth undersea reef that prevents large ships from approaching closely. Small boats or Viking longships with shallow draft, however, can row right into the lagoon, where the water is warm and the bottom is soft and sandy.
The only building on the island is a bamboo-and-palm-thatch hut, belonging to a crazy, muttering, Ben Gunn type castaway wizard. Apart from sunning himself on his balcony and unsuccessfully trying to brew potions out of kelp, the wizard tends a set of wide, shallow rock-pool-botanical garden of coastal and aquatic plants behind his house.
The wizard is harmless, and once he’s got over his fear of the adventurers he’s a mine of information about the island. The first thing he’ll tell the PCs is that he wants to rescue his friends but he can’t. Then he’ll show them some giant snail shells with membranes over the openings – these can store a couple of minutes worth of air, and allow for deep-sea diving. Then he’ll want them to try his potions (hoping to find one of water breathing). Finally he’ll get around to telling them the island’s secret, the reason why he and his friends came to it.
The island is actually a giant mollusc, or maybe two molluscs joined together.
The “lagoon” is a mouth with a ridge of stony, barnacled, tree-lined shell around it. The lagoon’s sandy bottom is the body of a giant snail-like thing loosely covered with sand and seaweed. This snail thing is connected to another that combs the sea floor for curiosities. The creature passes any indigestible curio it finds on the sea floor (bits of shipwrecks, Atlantean kingdoms) up to the mouth to get washed out by the waves, or to be rifled through by opportunistic murderhobos. Like the wizard and his friends.
So you could pan for treasure! But there’s a couple of complications. First, there are the crab-men – strong, violent and beautiful, with pearl-encrusted shells. They look like Kabutops, but covered in mother-of-pearl. Mechanically they’re like these guys. Those crab-men have taken to swiping all the treasure that gets washed up in the lagoon. The wizard has an uneasy truce with them – they let him live on his end of the lagoon, but they won’t let him in their end, where the floor of the lagoon shelves down into an opening deep into the mollusc shell – where they’re keeping the treasure.
And there’s another complication. The wizard came here as a member of a party of adventurers: where the others came for the gold, he had heard legends that an ancient and wise creature lived on the island, a creature that knew the way to paradise. The wizard brought a bunch of tools with him for exploring the island, including potions of water breathing, a diving bell, and a working magical submarine. He had to stay topside to power a ritual, to pump air into the sub. The rest of the gang went deep into the shell. And never came back. The wizard’s confident they aren’t dead – he tried casting Speak With Dead and although he got plenty of responses, none were from his friends. So he wants to know what happened to them, what they found, and what they did with his sub.
What the wizard doesn’t know is that deep inside the shell there’s a series of chambers, some filled with water, others with air. His friends did indeed make contact with the mythical creature – the mollusc, natch – in a small chamber right in the centre between the two great bulbous shells, where they found strange tubes that allowed them to mind-meld with it, and they are slowly being coccooned in pearl themselves. Mind-melding with the mollusc has some advantages – it allows you to control the crab-men telepathically, it feels really good, and it lets you into the mysterious and very slow-hatching plans of the mollusc, and of the party, who are now part of the same mental entity. It’s also very dangerous: breaking someone out of the mind-meld can hurt them, and anyway they won’t want to come away because they’re happy.
Indeed, the wizard’s friends are happy, and peaceful as long as nobody offers them violence, and willing to talk to strangers. They don’t care much about mere gold and jewels and rayguns and what have you. They’re looking for some special magical treasure, which the mollusc has been slowly amassing for years. They might be persuaded to help get the submarine for the PCs (using the crab-men), or drive the island to some useful location, or whatever you want. Maybe someone could try to mind-meld long enough to steer a crab-man to the sub, and then try to leave the meld to take control of the sub. But they emphatically do not want to be rescued, and they don’t want the wizard to join them in the meld – maybe they suspect he has some ulterior motive, maybe they realise their condition is a curse as much as a blessing, maybe they just don’t like him.
Past the mind-melding chamber there’s a narrow squeeze – too narrow for the previous party’s abandoned diving bell, but not too narrow for an unarmoured adventurer and a bag of potions of water breathing (emergency supply in the diving bell, which is probably still in the next chamber just past the squeeze). Beyond and below that, the chambers open out again into a crab-man settlement, with a whole city of crab-men hanging in ropy pearlescent strands off the bottom of the lower shell.
And tangled in the strands, the sub. Getting to the sub has one more complication – pressure. The bottom of the shell is deep enough to cause serious pressure problems for human divers (I don’t know how deep that is – look it up. I’m going to say about 100 feet). You could polymorph into a squid, perhaps. If you were confident crab-men didn’t love them some tasty squid. Or you could try to get to the sub quickly, since high pressure diving is all about time.
There’s plenty of treasure for a violent and uninquisitive party to take out of the island, but most of what can be reached easily is not very convenient: you could kill and loot the crab-men for as much mother of pearl as you can handle, provided you don’t mind it coming in foot thick slabs – that sort of thing. The crab-men also have a few valuables squirreled away in the top couple of interior chambers, hidden in a mighty tangle of everything the sea bottom could possibly provide. But to get to the sub, or the major treasures of the crab-men’s city, you’d have to get inventive. And to unlock the real potential of the island you’d have to get friendly with the mollusc.
Different campaigns will want the sub to have different possibilities and limitations. I picture it like a small version of the Nautilus from the classic Kirk Douglas 20,000 Leagues – big enough for 5 adventurers and their equipment, and capable of operating just as long as the wizard can sustain his continuous-air-making ritual (supplies enough fresh air for the crew and to power a small turbine for propulsion. The ritual must be conducted under an open sky – which is why the wizard can’t go aboard – and has a maximum range of 1 league).
Regarding the challenges of underwater adventuring, Harry Potter provided several magical solutions (gillyweed, which provokes a partial Deep One polymorph, outright polymorphing into some seaborne species, bubble-head charms, buddy-breathing with mermaids*). If needed the wizard could have developed anything from potions to a really long rubber hose. He does have a collection of diving-bell-like apparatuses around his hut, which he has grown out of nacre. These all have the disadvantage that they’re nearly impossible to move on land, being extremely heavy and having rough outer surfaces. Once in the water they become more tractable.
* OK, that last one is not canonical HP. But I bet there’s a fanfic.