|An advanced Arha noble from the postclassic era of their culture.|
Once upon a time, there was a civilization of wizards so powerful that they could hang their cities amongst the stars. Well, not really amongst the stars. More like amongst the Lagrange points in upper orbit around their world (your campaign world).
They encased their orbital cities in domes of glassteel on captured asteroids, locked them tidally, set up self-contained ecosystems with magically generated gravity, and devised permanent teleportation circles to allow easy passage to and from their manmade paradises. It worked quite well for several hundred years. Any time the orbital cities were threatened, they would simply roll a stone imbued with anti-magic over their end of the teleportation circle and wait it out. No teleportation circle meant no access to the city. It was an uncrossable moat.
Their problems were internal in nature. Bathed as they were in cosmic radiation, most children born in Arha exhibited deformities or mutation. In time, these mutations came to be highly prized by the inhabitants. Only the more normal-seeming Arha would interact with the people of the planet below, while the most inhuman Arha would remain in their city as its rulers.
Another significant issue would prove to be the city's undoing: the loss of magical expertise over time. The Arha retained great troves of their ancestors' knowledge, including how to make and repair the city's glassteel dome, but their warped physiologies became steadily less equipped to make use of it. Still, so long as they controlled the teleportation circle, nothing could threaten them from below.
A Soft Civilization Meets Hard Vacuum
Eventually, one of the orbital cities pissed off the wrong wizard. Maybe he was exiled from their midst for his lack of mutation and consequently had a better than usual understanding of the location of his city. Maybe he was just really smart. In any case, he devised and successfully prosecuted a method of piercing the glassteel dome surrounding the city known as Arha: he summoned a byakhee, rode it up to the city, managed with some difficulty to match its velocity and land, and thence disintegrated a significant swath of the dome.
The end came quickly: the atmosphere rushed out before most inhabitants could react. A few managed to escape through the teleportation sphere or by other means. Though the city boasted many precautions against this very event, including hermetically sealable buildings and so forth, most were long forgotten or in general disrepair. In any case, the magical acumen necessary to create glassteel was no longer present and repairing the dome without that would be impossible. The city went quiet and dark, a floating tomb.
|A Koros bird-lancer scans the horizon for foes.|
Eventually, knowledge of Arha was lost to the surface-dwellers and largely forgotten by the other orbital cities (who by now had their own problems to contend with). The Koros, a race of barbarous bird-riders, came to dominate the area around the planet-end of the portal.
The magic of the teleportation circle was quite beyond their limited arcane understanding. They knew only that anything that entered the area of the circle disappeared, never to be seen again. Eventually, the circle was incorporated into their religion. Bodies were lavishly prepared, festooned with such modest finery as their people could afford, and ritually placed into the circle.
As a nomadic, non-agricultural people already disinclined to sow seeds or waste wood on a funeral pyre, the ritual casting of corpses into the circle allowed them to totalize their taboo against digging in the soil. After nearly three centuries, the only acceptable means of corpse disposal is to deposit it into the circle.
Meanwhile, the bodies piled up in Arha. The same magical precautions taken to prevent teleporters from superimposing their bodies over air molecules also prevented the corpses from occupying the same space. New corpses landed atop a vast pyramid of vacuum-desiccated cadavers.
A little bit about the Koros: each man of the Koros takes at least three wives. Each of these wives can expect to have five or six children, of whom perhaps half will survive to adulthood. All of these that leave recognizable remains will be taken to the teleportation circle and deposited within. By the current century, there were hundreds of thousands of cadavers up there, lying quiescent and perfectly preserved by exposure to hard vacuum, just waiting for somebody to find them.
Eventually, the Necromancer came. The Koros tried to kill the obvious witch with their spears, as is proper in their culture. Their herbal tinctures that normally proved so effective at warding off magic did little against his swarm of animastrictor skelesnakes. It took him less than a month to killed everybody who might object to him declaring himself god-king of the Koros. He was the right person at the right time.
One big thing the Necromancer noticed very quickly is that there were not any graveyards or even particularized pyre-ash around. He's very in tune with these things, and would notice if they were there. Eventually, he got around to asking about it. The Koros explained. The Necromancer found the circle, sent a ghoul through the circle and back, and eventually figured out that the other side lacked pressure or atmosphere.
Natural physics are not usually in the Necromancer's wheelhouse, but he still managed to devise and construct a primitive bathysphere. With his zombies carrying it about, he has explored the ruined city of Arha. What a magnificent discovery! What a wonder! What a necromantic tinderbox just waiting for a negative energy spark!
The circle led to the perfectly preserved corpses of two whole cultures: the relatively small number of Arha and the vast heap of Koros. Animating them would become the Necromancer's magnum opus.
|A Koros mummy that has soaked in cosmic energy.|
Things From The Broken Dome
The mutant corpses of the dead Arha possess many characteristics that make for powerful undead: extra limbs, maws in their torsos that whisper unspeakable secrets even in death, gravimetric muscles that allow limited telekinesis, and so forth.
Nor are the Koros mummies entirely devoid of interest: bathing in cosmic radiation for centuries has imbued many of them with a smattering of strange qualities: polychromatic auras that becloud the mind, achrononistic ways of interacting with time that allow them to blink in and out of existence, claws dripping with void-ichor, and so forth.
Nor is the dome solely a necromantic treasure. The Arha were a wealthy people, their coffers filled from selling maps and intelligence gleaned from telescopes and their orbital vista. They also possessed a goodly number of magical objects inherited from their ancestors. There are batteries containing raw sunstuff, rings that allow soaring through the air, and more.
There is also the Nullification Stone, a 3-foot sphere of stone that nullifies magic (as an anti-magic field) in a 5-foot radius. According to my math, a 3-foot sphere of granite weights over 19,000 pounds. I have no idea how the Necromancer would use this or what kind of special undead creature he might design to house and wield it, but I'm sure you can come up with something.
Other trinkets include "exo-bracelets" that allow survival in a vacuum (lying forgotten in a vault), ioun stones that reduce your need to consume food by an order of magnitude, and damier-print cloaks that halve damage from projectiles via inertia manipulation. There are also scrolls detailing astrophysics and primitive spelljammer ship deck plans.
Perhaps most perilously, the Arha libraries include well-encoded books describing the rituals necessary to snare a large asteroid and maneuver it into proximity with your campaign world. The ritual poses an existential threat to humanity should it ever fall into incompetent or malicious hands. The Necromancer doesn't need it and might just sell it (perhaps to the Ishnindah).
A Dark Future
Conquering the world seems so plebeian, but that's mostly because nobody ever manages to do it. Actually conquering the world or even just a significant portion of it seems pretty appealing, particularly if you have the means to achieve it. When you have a hammer, everything looks like a nail. An undead army numbering in the hundreds of thousands will probably suffice for at least a continent.
The Necromancer has the Koros, too, who are largely immune to magic (though not to summoned or animated creatures). Their bird-horses make for agile, deadly cavalry, leaping into the air for twenty feet at a time and crashing down onto enemy troop formations. The Koros have decided that the Necromancer is the sacred representative of the afterlife. After all, he has completely mastered the power of the teleportation circle that leads there. He is even capable of returning things from the other side!
Really, the only plausible force for stopping the Necromancer's army would be some kind of crusader order capable of dealing with diverse undead menaces. Which is where the Koros come in: if the Necromancer gets even a whiff of paladins, he'll dispatch a hundred bird-lancers to take care of the problem. Or a thousand. Or ten thousand (there are a lot of Koros). This meshes pretty well with their values, they've always wanted to conquer and destroy the decadent soil-cultivating cultures that sprawl across the world, but largely lacked the means. Until now.
I'm sure you can take it from here. Good luck with all that, player-characters.