History
World Weep Opening History
During the genesis of the Worldweep and Mathesis's rise, the underground tunnels of Golarion were in turmoil: The unstable movements of fault lines caused many Darklands creatures to flee to isolated caverns for safety and all number of titanic creatures (such as the fearsome cave worm) raged across the larger tunnels.
Age of Collapse History — Era amongst the Dead
In Garund, though, it was a different story: The Eye of Abendego found itself emboldened, and the perpetual hurricane overwhelmed the Sodden Lands. Though the storm has since lessened in ferocity, those that fled into the underground continue their day-to-day existence within its tunnels. The shisk tribes have each established their own enclave in the Darklands across the Mwangi Expanse along with pockets of kobold warrens who have found themselves imprinting onto the power of cavern monsters, such as adamantine dragons and various aberrations.
The most significant Darklands region worth noting is that beneath the Spellscar Desert. In the centuries following the creation of the Worldweep, the dwarves of Dongun Hold found themselves in a desperate scramble to escape storms of extreme magical radiation that seeped unnaturally deep into the earth. Instead of a "Quest for Sky", it was, instead, a "Quest to Escape the Sky". Several small bunkers were built, serving as protection from the chaos out on the surface. But despite their best efforts, or perhaps because of them, the dwarves who fled underground never returned to the surface. Abandoned—or seemingly-abandoned—dwarven bunkers have been left scattered across the Mana Wastes. It's said that some contain wonderous artifacts of engineering, while others are nothing more than trap-filled dungeons with irradiated monsters. Navigating one of these bunkers is the easiest means of traveling into the first level of the Darklands.
There was one bunker, however, that seemed to grab the attention of the surface world. The central bunker of Dongun Hold's dwarves lays close by in proximity to Moonlit Junction. Dwarves from the region are likely to have hailed from this bunker; unfortunately, these dwarves are unaware of the fate of their brethren. This has motivated some of them to launch expeditions into the other bunkers.
Age of Renewal History
Beneath the desert is an interconnected network of branching caverns, forming a web of paths to travel. Attracted by the pure magical energy spilling out across the Mana Wastes are the beings known as surki: Despite previously congregating around the Sarkoris Scar, the diaspora of insect creatures began to take root within the former's underground. Unlike most people living in the area, the surki seem capable of navigating the hazards of both the tunnels and the dwarven bunkers that serve as gateways to them. 75 years ago, the Marked of years past had a found a way to bond with highly-mobile beetles (then aptly-named "speedles"). This practice began to spread, and the surki of the Mana Wastes Darklands honor this tradition. That said, there is one oddity about their "speedles": Instead of being the size of a small horse, they are instead the size of elephants capable of traversing miles of caverns in the span of hours. Despite their admirable speed, the dangers of the Darklands are ever present. It is not uncommon for explorers to see blue and green lantern lights of a speedle caravan, which have slowly become a popular form of travel in such dangerous areas.
Government
The governments of the Darklands are unique to each community within, mirroring the typical hierarchy and dynamics of power and infrastructure as tribes and kingdoms of the surface.
Other notable changes from the past Golarion
Common ancestries: Non-Descript Cavern Elves (formerly known as Drow), Hyryngar Dwarves, Surki, Fetchlings, Kobolds, Goblins, Gnomes, Fleshwarps, Shisk, Minotaur, Ratfolk.
Threats: Cults to unholy gods or elder beings, cave worms, various predatory beasts, adamantine dragons, aberrations, cave-ins, earthquakes, floods, undead from antiquity.