I started with the idea of some a trio of Jedi whom the Empire has allowed to live since they are holding back something Cthulhu-like. I figured a water planet would fit well for that. Then I began drawing from Gargantia on the Verdurous Planet to fill out details about the planet. And I kept drawing on it. Pretty soon my Lovcraftian idea went full-on anime adaptation. Still think it would be fun, though. It takes place before A New Hope.
There are a trio of old Jedi on the ocean planet of Rilleth in the Lavicroft system.
Rice Beren, Jafan Desyk, Soont Corren. They are the Keepers.
They are part of an order that holds back an entity called the Devourer - one of them must be focused on meditating at all times. If the meditation is broken for more than a minute the Devourer will be released. They keep a rotation. Before the Purge they were sent new Jedi to train as replacements every five years. Now they are old and there is no one to replace them. While the Empire has allowed them to live they have forbidden them from training new Jedi to act as replacements.
The Keepers have about 100 servants both in the temple and located in other places to acquire supplies and other forms of support. Most outsiders just think of them as an odd but harmless religious group as they do their best to keep the Keepers and the Devourer secret.
They live in an ancient temple that sits above the water - it is actually the tip of a pyramid that descends 10,000' into the the ocean. The base is almost 20,000' wide. It is said the Devourer sleeps inside and barely fits...
The Empire knows about the Keepers and keeps them a secret. Rilleth is one of the few planets that is a source of Cortosis ore - oddly the underwater mines make it safer to extract. The Keepers have been contacted by the Empire and informed that they will mostly be left alone and get the Empire's support as long as they don't train any new Jedi. They Keepers are not happy about this but the don't want to see the world destroyed.
Since there will be no more replacements for the Keepers the Empire is building a massive ion weapon to use on the Devourer when it inevitably awakens. It is powerful enough to short out a super star destroyer with a single shot.
Cult of the Devourer
The existence of the Devourer is relegated to myth. Some people take it quite seriously, though, and want to see it freed. This is due to exposure to a biological agent that affects the subjects' minds in odd ways. This agent was created by the Biologists long ago as a weapon against the Colonists as it causes self-genocidal impulses.
Cultists are loathed by almost everyone as they are quite clearly crazy, mostly suicidal and often homicidal. Since they kidnap people and warp them into their beliefs somehow (by exposing them to the agent) they are often hunted and prosecuted everywhere. They are always trying to find followers of the Keepers so that they can find the temple and free the Devourer.
Rilleth at one time had an advanced civilization but they suffered too much ecological damage and most of the planet was submerged by the oceans. It seems some portion of the population managed to get off planet but suffered extermination not too long afterwards.
There are currently no known native sentient life forms on Rilleth. There are the glowsquid - four tentacled, phosphorescent squid-like creatures that are about 2 meters in length. They leave glowing trails through the water that lasts for days. These trails also provide excellent fishing. They are known to have nests deep below but no one has bothered exploring them as they turn hostile when you get too close.
The ocean floor is littered with the remains of the previous civilization. These are obviously found at the highest points where land once used to be. However it is rumored that the most advanced technology lies deeper.
The other planets in the Lavicroft system show some signs of ancient colonization and space stations exist but they all seem to have been damaged or destroyed in combat. Additionally, massive, artificial, hive-like structures have been found floating in space nearby many settlements and also show signs of battle damage.
Many different people have come to Rilleth over the centuries and established a rugged civilization. Some were scrappers, some were treasure hunters and some were just trying to get away. There turned out to be little profit in the scrap available but the occasional treasure kept people coming.
A few cities exist where mountain ranges still peak over the waves. These are the closest things that an outsider might consider as proper civilization, complete with spaceports and holonet access. These places are under Imperial control.
After the cities are the Gargantua - massive fleets of ships and floating platforms that have literally banded together into a collective society. They are rough places that survive on scrapping and hydroponics. Individual fleet captains act as a city council of sorts and may choose to leave the Gargantua charter by taking their ships with them.
Finally there are the pirates and independents. The former often band together like Gargantua but rarely reach the same size. The latter rarely last very long without forming alliances.
Port Pashna - The largest city and primary spaceport on the planet. The tallest mountain on the planet was leveled to make a flat surface for the city. It is under complete Imperial control and well ordered. The planetary governor tolerates the existence of some of the more reputable Gargantua and they often come here to trade. The primary Cortosis warehouse and shipping center is also here.
Thalu Cove - A smaller, seedier city that has the only other public spaceport. Located inside of a mountaintop that was hollowed out. Not exactly a pirates' den but definitely a smuggler's paradise and place for more questionable transactions to take place. The city has an Imperial governor but he is corrupt, taking kickbacks from pirates and smugglers to turn a blind eye.
Hordo's Nest - Hordo Tetsu, a Rodian self-proclaimed "pirate king", hollowed out a mountain that had a tip jutting out above the waves and built his base beneath the water. Initially he used it to hide his fleet but over time it became an illegal trading center for many different pirate fleets to meet. Aside from trading with themselves that have "runners" who take merchandise to more reputable places for fencing. While Hordo doesn't technically command any fleet but his, everyone respects him and would likely answer a call to action. Ruthless but rationally business-minded.
Cortosis mines - There are a dozen of these all over the planet. They are heavily guarded by the Empire in the air, on and under the water. Massive hydraulic jacks bore into the earth, collect the results into a water pump that reaches a filter part way up that separates out the Cortosis and then pumps that up to a refinery platform up above.
Aneen Dal - The largest of the Gargantua and the reason that type of collective was given that name. At it's peak, Aneen Dal was larger than Port Pashna. Founded over a hundred years ago by a Mon Calamari of the same name, Aneen is considered to be one of the points of light on an otherwise miserable planet. There are over a hundred fleets tied together and their council is more like a parliament with a Chancellor, Carmiya Kale, as head executive. They even have a small spaceport with room for two frigates and some fighters.
The Temple of the Keepers/Devourer - Technically the same building. The tip of the Devourer's pyramid is where the Keepers and their attendants live. The temple is actually made of an unknown alloy using Cortosis as a base and is gleaming silver. A single Jedi meditating can activate the Cortosis-like properties in the pyramid that apparently keeps the Devourer at bay. It is thought that since there would never be enough Cortosis to make a cage large enough that it was spread out with this alloy instead.
The Imperial Weapon Outpost - Not far from the Temple is a massive Imperial base that sits on the water. Aside from the troops who are dedicated to keeping the Temple safe and unknown there is also a massive ion weapon that is being designed to kill the Devourer. The idea is that since the Devourer seems to be energy based enough that Cortosis can contain it, a large enough ion blast would short it out long enough for its biological components to die. Anyone who sees the weapon may notice that it is aimed at the water level and not at space...
Rilleth was advanced technologically but hadn't yet achieved hyperdrive. The ecological disaster was going to submerge the planet no matter what and the civilization was divided on what to do about it. The Biologist faction insisted that genetically altering themselves to live underwater was the answer while the Colonists wanted to colonize other planets. The Biologists told the Colonists that terraforming wasn't possible yet and life in inhospitable space would be a miserable existence. The Colonists told the Biologists that there was no predicting how the changes would affect them and feared turning into monsters.
These two sides eventually began warring. Once the Colonists successfully began leaving the planet the Biologists began modifying themselves to survive deep space and following after. By this point they had become somewhat monstrous. Colonists became so militarized that no one could remember what freedom was.
The biologists began to prepare an ultimate weapon, a massive entity that was nearly indestructible and powered by a fusion core. A sort of biological robot that could be constructed instead of grown. It was completed but never activated as the war in space had nearly petered out as the Colonists were destroyed to a point where they would never survive and the Biologists had warped themselves to a point where they barely had any sentience left.
The Old Republic first contact team arrived not long after this was over. Only a few Biologists with any intellect remained and they weren't able to warn off the explorers before they accidentally awoke the Devourer. By cooperating with the remaining Biologists they were able to put the Devourer back in its cage and learned how to use the Force to maintain the Cortosis alloy once the last of the Biologists passed away. Since the Devourer could survive in deep space they had no desire to let it loose, even if the planet was essentially uninhabited at this point.
The PCs could have any number of reasons to come here. If they're of the shady variety they could simply be picking up a Cortosis shipment for smuggling. If they are bounty hunters they can get wind of the Jedi's existence and hope to collect the huge general bounty on surviving Jedi. Rebels would likely want to recruit any surviving Jedi. An Imperial officer not in the loop may send some agents to collect the Jedi for a reward. Either way the PCs should eventually have to deal with what to do about the Devourer and the surviving Jedi.