In elder days, the Empire of Zahar ruled across the lands of the Inner Sea. The literal children of the moon, they build great crescent-shaped monuments to their dark and chaotic gods, and their sorcerer-lords created the beastmen as their servants. In time, the race of Men came to the Inner Sea (from where is disputed - the Skanucks claim that men fled from the Farthest North to escape the Long Winter, while the Urdukhar believe they sprang fully-formed from the river mud and the Islanders claim to have always been upon the isles). For an age their fledgling kingdoms paid tribute to Zahar in maidens and slaves, but the sacrifice of the king of Soros' daughter finally sparked the Zaharan War. Unable to crush the humans with their legions of beastmen, the Zaharans researched rituals too unspeakable even for them, bringing a cataclysm down upon their heads. Their lands burnt, temples shattered, and legions decimated, with hordes of angry humans after their heads, the surviving Zaharans swiftly made themselves scarce.
What once was the Zaharan heartland lies well west of Dardantus. Once a fertile river valley of sandy soil, it is now a mix of vile mires, scorching deserts, and crazed glass where the cataclysm melted the sands down to an unknown depth. Terrible monsters created by the Zaharans roam the land and have established breeding populations, with bands of beastmen as their primary source of food. The dead walk, unholy things stir in ruined temples, and when the weather's bad it rains blood and the winds drive shards of glass at deadly speeds. It is a hellish place, where even the bravest men dare not tread for long.
Between Dardantus and Zahar, the land suffered a great upheaval rather than being burnt, forming a region of badlands, marshes, and stunted forests. These are the Shieldlands, so called for lying between Zahar's horrors and the rest of the lands of man, and the folk who settle here are a heterogeneous and surly bunch; outlaws, monster hunters, charlatans, relic seekers, cultists, trappers, petty warlords, prospectors, and exiles from a thousand lands. Few people migrate to the Shieldlands willingly, and wisely so - it is said that no man has ever died of old age in the Shieldlands, and not without reason.
Politically, the Shieldlands are not unified under any central authority. There are a number of small forts (as well as many ruins) and "cities" (which would be called small towns in any other region) scattered through the Shieldlands, but who rules each changes fairly often - assassination and "unfortunate accidents" are not uncommon. The terrain has so far impeded the unification efforts of a number of ambitious warlords, and most schooled in history posit that unification is not feasible. Notable among the towns are Gold Rock (named after the discovery of gold there following the upheaval, now a thriving market town), Freetown (which maintains a citizens' militia and refuses to be ruled; has the best pies in the territory), Dusty Hole (founded as a temporary cave shelter during a duststorm; turned out there was clean water in there and it was pretty safe), and Dirk Hill (site of the infamous Dirk Hill Massacre and a popular place among duelists and sellswords).
Arable farmland is uncommon in the Shieldlands, but that doesn't stop people trying to make a new life from scraping subsistence out of the rocks, sand, and scrub. Homesteads are a common but risky mode of living; if the goblins don't get you, your neighbor might. Most Shieldlanders tend to sleep with a weapon close at hand as a result. Goats are popular as far as livestock goes, and hard liquor is the preferred beverage (with "Anything with alcohol" running a close second). Water in the Shieldlands is often unsafe to drink, and the contaminants that blow in from Zahar will do less pleasant things to one than a mere case of loose bowels. Drinking, wenching, drinking, gambling, drinking, and goblin-hunting are the regional pastimes, in order of decreasing preference. Duels are also a common feature of life in the region, though one feature of Shieldland duels that surprises many recent immigrants is that there are no rules (for all practical intents and purposes), though it is considered polite to verbally challenge the target before trying to stab them, and downright gentlemanly to wait for a response and let them draw a weapon first. The Shieldlanders tend to wear greys and browns, typically as a result of lack of washing, and also favor a peculiar style of long leather coat. Masks and bandannas to keep the dust out of one's lungs are common, too; identity concealment is a nice bonus for many wearers. The Shieldlanders are not particularly devout as a rule, tending to pray in short, violent bouts to any deity that will listen. As far as clergies go, the Ashtari blade-dancers are by far the most popular in the territory, though it is fortunate for them that they are well-armed and well-trained. Various chaotic cults have homestead-shrines as well, but no mainstream religion has many worshippers among the Shieldlanders.
Common threats to the Shieldlanders include beastmen (most often goblins, but frequently also gnolls, orcs, and worse), undead, werecreatures, and eldritch monstrosities like skittering terrors and beetlephants. The typical response to a goblin raid is to form a posse and go after them, but skilled slayers of undead and werebeasts are always welcome in many homesteads. The monstrosities tend to attract trappers seeking to catch the beast alive and sell it to the fighting pits of Dardantus; such trappers are regarded with suspicion by all right-thinking Shieldlanders, as they've been known to be a bit unscrupulous with their choice of bait. Zaharan ruins and ruined settlements are both fairly common in the Shieldlands, but most wise folks (read: not PCs) avoid them.
As far as cultural composition goes, the majority of Shieldlanders are Myrmidian or Sorosi in origin. The Myrmidians are often failed or rogue mercenaries, while the Sorosi are washed-up pirates, bandits, con men, wandering swordfighters, and the like. A reasonable portion of the 'civilian' population is a mixture of these two ethnicities, and consider themselves to be native Shieldlanders. The granite dwarves are fairly well-represented here as prospectors, merchants, smiths, and warriors; their craftpriests are also well-respected for their usefulness, lack of proselytizing, and ability to hold liquor. The elves, Urdukhar, and and Skanucks are all uncommon but not unheard-of, while the Sajuk are quite unfamiliar. Also notable is that the Shieldlands are the ancestral home of the hobbits; they keep to their shires and defend themselves via savage guerrilla tactics, having long ago had any innocence burned away by goblin raids. They do, however, produce an exceptional brandy which is widely beloved throughout the Shieldlands, and many a town has gladly welcomed a family of hobbit refugees on the condition that they take up brewing and distilling it.
Prominent inspirations for this area include Renegade Crowns and the American Old West. It's one of the least Ancient World-y places on the Inner Sea, but it sounded like a lot of fun at the time (and is perfect for ambitious PCs to trying to set themselves up as lords).