The stealth ship is of Djinn origin, created as a test platform (and VERY VERY expensive, as it is made from transmuted water ice from where the plane of air meets up with the plane of water-- solid matter being exceedingly rare in the plane of air) to test a prototype system to hide a planned invasion. (population crowding, coupled with competition from migrants from the material plane inhabiting the metal spheres for what material provisions the plane of air has available, has driven a group of djinn to seek a chunk of the material plane for their own permanent residence, and they dont care to be discovered until their main ship is ready), before the experimental protection system on board is tested in the "Target environment". Hilariously banish themselves to avoid other hellish consequences as the ship malfunctions. Wish to avoid being detected *AT ALL COSTS*, and utilize captured material plane race host bodies from the populations inhabiting the plane of air to use as disguises. Have installed "facilities" for themselves in this incognito form. (this is why the test ship is more or less human operable, and furniture/appliances are suited for typical humanoid races-- includes such useful contrivances as a magical equivalent of a sickbay, to maintain their host bodies' health with. All references to the origin of the vessel are purposefully missing, as they are not messing around here. No divine, auran, or ignan script is present on board, and it is conspicuously decorated in material plane styles. After banishing themselves, the dazed, confused, and amnesiac victims of their possession awake on a crashing ship they have no idea how to pilot, with all manner of magical chaos going on around them, and they make a break for it, and jump as soon as they see something they think they can survive landing on below.
Their ultimate plan is, as stated, to claim a fair chunk of the material plane for their own purposes-- and have constructed a floating cityship of colossal size in the elemental plane of air over the course of a very protracted period of time, using very high mastery of air magic and transmutation of the most plentiful source of physical material in their plane, water collected near the border with the plane of water. Given the lack of resources, strong competition from the shorter lived (and more fecund) material plane dwellers infesting the plane of air, they are living a rather claustrophobic, tightly packed existence on their (very elaborate) flying city, which they intend to displace wholesale to the material plane, and set up residence, in obscurity high up in the sky, collecting the very valuable raw materials of the material plane, and transporting them back to the plane of air, to enable them to grow as a dominant power there.
Having very little experience with the material plane, and not being nearly as skillful with abjuration type enchantments, their test platform is seriously flawed-- and malfunctions spectacularly, forcing their self-banishment, and abandonment of their material plane host bodies, back to the plane of air, where they report the failure.
Since this is part of a vanguard that precedes an invasion, there are other agents already in the material plane using host bodies, serving as intelligence agents, who keep the cityship informed, and are charged with damage control concerning the failure. (exposure of the plan before they are able to send a very large invasion force undetected would result in organized resistance to the plan, which the djinn simply dont have the material resources to engage in. Their's is a stealth victory scenario where they appear in-force, suddenly, after having been silently utilizing the material plane for an extended period after setting up secret bases of operation to facilitate their plan.)
Needless to say, it all goes horribly wrong with their test vehicle, and it crash lands. The accomodations on board are practically just damned perfect for professional adventurers, because it has everything an adventurer could possibly want on board-- but is INSANELY magical, and has a high magical proficiency cost associated with unlocking some of the more advanced features of the vessel (such as mass matter transmutation based resupply of raw materials/reagents from common feedstocks (water, etc), use of the healing chamber, living quarters, etc) In terms of size, it has the footprint of a large yaht. It is designed to effortlessly dock with the city ship, and has a means to corporeally transfer itself and its occupants to and from the elemental plane of air (when functioning properly, and controlled by a very competent user of elemental air magics) This makes it a perfect "counter-infiltration" device for thwarting the diabolical evil plan.
Again, many of the systems on board are not operating "properly" due to not being native to the material plane, and not created by minds shaped by the material plane, and its rules.
Basic outline of plot arc:
Ship starts crashing- party detects massively powerful magical artifact that overwhelms any of their magic users' abilities to sense magical items.
Party begins searching for the source of the anomaly, runs into random encounters with treasure hunters and adventurers, who are likewise attracted to the anomaly.
Party gains access to the crashed ship, gets assaulted by all the insane magic going on onboard. Either succeeds or fails to appropriate the vessel, and get it working.
Plot arc (A):
Party gets the ship working--
The party gets the ship working, and goes on a wildly successful treasure/adventure hunting campaign, given that they have an all expenses paid form of rapid transit, resource storage, free resurrection/healing services (if required feats are met), and ready access to an inexhaustible supply of magical reagents (if required feats are met)-- If they get the stealth system working, it functions as a very safe place to stash loot. Party gains notoriety, and at DM's discretion, malfunctions of the vessel happen, causing chaos at inopportune times-- and the forces of the invasion track their exploits, and predict their next destination.
The vanguard forces attempt to re-appropriate the vessel from the party after the party reaches appropriate skill level. (Since these are djinn in disguise, you do the math on that.) This leads to conflict, which ultimately reveals djinn involvement.
Party explores the djinn connection. Discover hidden "displacement drive" functionality.
Arc (A1)
Party decides to assault Djinn cityship, and stop invasion. Ship permanently docks with cityship, and is lost when the cityship is destroyed. (Djinn can fly, so the destruction of the cityship is just a major inconvenience to them, rather than totally destructive to their political group. The precious raw material of the cityship is not lost, just not in a useful whole anymore. Causes a many-century long hold on any invasion plans while they rebuild their citadel.)
Arc (A2)
Party decides to assault Djinn Cityship after more completely studying their stealth ship's docking systems, finds way to prevent the ship from being remote controlled upon docking. (this is what causes the permanent docking in A1, as the docking moors refuse to disengage, as a launch is not authorized by the central govt, and all the controls are locked out.) Makes escape on the stealth ship, retains possession.
Arc (A3)
Party decides the Djinn invasion is not their concern, dont want to risk it. Try to keep the stealth ship-- Increasing resistence from vanguard forces either ultimately overwhelm the party, or destroy the stealth ship.
Arc (A4)
Party decide that they dont want to keep the stealth ship, but also dont want to go unravel evil master plans at enslaving a good chunk of the material plane. Decide instead to either destroy the stealth ship, or rig it to return to the city ship armed to self-destruct, or otherwise cause magical calamities upon docking when they are not aboard. This severely damages the cityship, but does not destroy it. Pisses off the djinn, who then hunt them relentlessly. If they choose to just destroy the ship, still hunted for "knowing too much", which jeopardizes the plan. Ultimately have to get to the material plane of air ANOTHER way, find a way to infiltrate the city ship, and stop the invasion force WITHOUT the aid of the stealth ship. Endlessly random encountered until they do.
Arc (B)
Party fail to salvage the stealth ship.
Vanguard forces observe and monitor them, to see if they learned anything about their evil master plan. Various hints are offered, until convergence with Arc (A4), "Hard way" option is possible, then proceeds that direction.