The Missing City: Rathania
by WingofCootPopulation: 10,000
Rathania is named, along with Reophis and Nuathys, among the Dead Cities in modern Deshret, and is memorialized alongside them. But it is not dead, though it has vanished from the face of the planet.
The small city state of Rathania originally stood near the center of Deshret. It was about the size of Pflaaopolis, and like that city state was heavily devoted to one Immortal - in this case Rathanos. But unlike Pflaaopolis, which was totally cleric-ruled even in Deshret's early ages, Rathania's rulers were chosen from among its leading mages, though the clerics of Rathanos controlled much of the day to day function of the city state.
The ruler of Rathania, Lanatenti, was a mighty wizard even among the mage-lords of pre-imperial Deshret, and while she knew much of the ancient war magic, she also knew its cost. She left the council of war that prepared to attack the Imperials on the coast, and called together the greatest mages of her city to hastily prepare her own solution.
The great magic of ancient Nithia could be used for more than destruction. Rathania vanished from the surface of Vaniae, becoming its own plane. Those who sought its ruins afterward found only a crater.
Rathania is now in a plane which consists only of its 12-mile-diameter oasis valley and surrounding cliffs. At the top of the cliffs, an unbroken roof of mist shrouds the valley - shining during the day, only dimly shimmering at night. Anyone who tries to fly, levitate, or otherwise move into the mists finds themselves emerging again below the mists, now moving in the opposite direction they intended.
Time in Rathania's new plane does not pass as it does in the Prime Plane; the rate is only 1:20. Thus only 32 local years (about 60 Mystaran years) have passed for the inhabitants. They still represent a pre-Imperial, and even pre-Union, Deshri culture - one much closer to Mystara's original Nithians. The religion is centered on Rathanos, and its elemental aspect holds Fire as primary, due to its connection with Energy and magic; rather than the stricter theocratic model that has developed as a way of preserving the Deshri under Imperial rule, it promotes creativity and change.
With the benefit of magical life extension, Lanatenti still rules over the dislocated city-state. She appears physically middle aged but in fact is beyond the normal human lifespan (about 130 normal years or 70 Vaniae years). Due to her role in saving the city and her political genius, and over the decades of isolation, her power has increased from the lifetime head of a council of mage-lords, themselves powerful both individually and through their families, to an essentially absolute ruler. Effectively, Lanatenti's total rule provides stability and order to counterbalance the chaotic nature of the religious influence.
Rathania's departure from the Prime was rapid and left little time to plan. While, like all Deshri oasis city-states, it was essentially self-sufficient in food and consumable goods, it was not entirely independent of trade either. Rathania had no good sources of metal, and none of its spellcasters had spells to create it. Rathania is rapidly becoming poor and technologically backsliding.
Lady Lanatenti has, through magical means, learned the state of current Deshret. She believes she possesses the ability to reverse the change. Both these pieces of information are kept secret from all outside her innermost council.
She realizes that eventually it will be necessary to return Rathania to Vaniae, but this will simply lead to its conquest by the Empire. Her current dilemma is exacerbated by her research into Immortality: clearly the separation of Rathania into its own plane was not sufficient to achieve it, but she cannot make any great achievements within this tiny plane where she already rules. On the other hand, Imperial Deshret has no room for a conquering wizard empress.
[I imagine Lanatenti as somewhat of a "Galadriel if she'd taken the One Ring" character - ruling by both great magic and great personal charisma, as well as political skill; magically enforced loyalty is probably a known & accepted requirement of joining her inner circle, and she likely has a personally researched, unique Geas spell variant for the purpose.]