Ascalhorn

Start here: http://forgottenrealms.wikia.com/wiki/Hellgate_Keep

Then, modify as such:

• The wards the Harpers placed in 886 created a wall around the city, about a mile from its walls, which prevent passage by any creature of fiendish blood, in either direction, whether physically or magically, and moreover block any magical effects they attempt to pass through the wall.
• Much of the population had died or fled in the ensuing war, but some remained. Those who could escape through the ward did, leaving a small core of tiefling citizens behind (a few hundred, nearly a thousand, mostly upper class, as they considered fiendish blood a "highborn" trait). The city had been home to tens of thousands prior to its fall, so it maintained an eerie emptiness for generations.
• The small amount of arable land was enough--with the help of magic, which the "highborn" had plenty of--to sustain their physical needs. The city had been built upon a source of fresh water, so that wasn't a problem. For the most part, they simply continued on, begrudgingly taking up occupations formerly staffed by their "lowborn" slaves and servants to keep their lifestyles in order.
• The arcanist Wulgreth, who'd driven Ascalhorn to its sorry state, was long gone. The power vacuum produced a number of short-lived rulers, until they yielded to Hannibal Kaine, father of Balduras Kaine, the city's current lord as of 1385. The House of Kaine was not noted for their mastery of arcane arts, but for sheer physical power, backed up by their copious concentrations of infernal blood.
• It's worth noting that the tiefling citizens of Hellgate Keep (nobody inside the city calls it that, btw, it's still Ascalhorn to them) don't have leathery wings, tails, and the like. When you're accepting fiend blood to become super-human, you might question it if that's the outcome. Due to the particular nature of their bloodline, their physical manifestations are minor--more serious ones are rare, and usually culled at birth.
• Moreover, they don't think of themselves as fiend-blooded. They refer to the source of their "highborn" status as "The Lightbringer", an angelic entity, whose "blessing" makes them long-lived (even immortal, for those with the purest blood), stronger, more healthy, more sharp-witted, more capable with magic, and of course, more comely, as would befit their nature.
• The ruling lords of the House of Kaine know the truth, of course, as do most of the city's arcanists and scholars. But it simply wouldn't do to discuss such things openly.
• For the first few decades after the wall went up, Hannibal Kaine focused his efforts on escaping the barrier, and wreaking terrible revenge on his perceived enemies. He drove the arcanists like slaves to find ways to escape, and punished them severely when they failed, imperiling his city's slim reserve of magical ability. When his son "succeeded" him, he did so with the secret backing of the arcanists, scholars, and former ruling elite. He then proceeded to back off on the fruitless escape plans, and rule quietly, often in solitude, for rather long stretch.
• In 1122, a party of adventures penetrated the ward. They were mostly slain, but one survivor, Lucia Ashkali, was chosen to live, and pressed into various services by Balduras.
• In 1123, Lucia bore him a son, Adrian. In 1126, another son, Tiberius. Their birth was a deliberate effort to produce scions of his power who were human enough to pass through the wards and do his bidding beyond. This proved successful, and they went on several forays into the outer world as they grew.
• In 1142, Tiberius rebelled against his father, with the backing of his "uncle", secretly an arch-fiend in service of their family's ultimate infernal lord, who wished to break the stalemate and make use of his investment for a change. Lives were lost, including Tiberius' mother, but he and his brother escaped the city, never to return.
• Few know of the events of the next two centuries, but the story continues thus...
• In 1364, the House of Kaine was long gone, and the city was ruled by one called Grintharke, a cruel, visibly-tainted tiefling lord. A band of adventurers entered the city, and, after being imprisoned and cruelly tortured for months, were thrust into gladiatorial combat with Grintharke for his amusement. This proved foolish, as one of them, armed with the Shattering Swords of Coronal Ynloeth, slew Grintharke (and himself), plunging the city into chaos.
• Word of the chaos trickled out, and a number of expeditions into the city were mounted, some right-minded, some devious. The most important result of this was the conquest of the city by chieftain Tanta Hagara and her Blue Bear tribe of Uthgardt. She was "wed" to one Kaanyr Vhok, who masterminded an operation to consolidate her power and use her barbarian horde as a weapon beyond the city.
• Ultimately, this resulted in the intervention of powerful adventurers (who totally weren't Harpers) who used the Gatekeeper's Crystal to destroy the wall. The ensuing magical tidal wave leveled structures, slaying virtually everyone in the city. The fiendish-blooded were banished to the Hell that bore them, while the mortals were crushed by stone and burned by magical fire. The city remained a ruin for years.
• In 1375, adventurers learned that the city wasn't as dead as it seemed. Wulgreth, the arcanist-turned-lich originally responsible for the city's fall, was harnessing the power of the site to cook up an army of undead and demons for what could only be assumed to be a terrible purpose. He was ultimately thwarted when the adventurers--with the help of an ancient clan of song dragons--recreated an ancient High Magic ritual to truly, finally banish the city and its lands from Toril, at terrible cost. The city and its grounds were not destroyed, but rather removed entirely from existence, replaced by a verdant expanse capped with a massive tree, overflowing with life energy.
• In 1385, Toril and its sister-world Abeir crashed into one another during the Spellplague, and, as if it had never left, Ascalhorn returned in its full glory, shining and bright, and populated with thousands of "highborn" tieflings. Having no land to occupy, it simply hovers in the sky on what looks like a flattened, inverted mountaintop, about a mile in radius. It is currently ruled by a High Arcanist, who has claimed the city's manifest destiny is to restore the might of ancient Netheril, with themselves at its head.