The most signficant event of the entire 11th Age was the emergence of the Duergar. The Duergar were heretofore unknown to the surface world, even to the dwarves. The dwarves, in fact, were overjoyed to make contact with them. The Duergar initially presented themselves as a lost clan of dwarves, cut off from main dwarven society and culture during the exodus from Underhome millennia prior. (That probably actually isn't even a lie.) They look like regular dwarves, except with ashen grey skin and red eyes that glow a little bit in the dark. They're adapted to live in the Underworld, like the Mind Flayers and some kinds of Drow; the poisons down there don't affect them. The dwarves welcomed them back with open arms. The Empire was a bit more skeptical, but the Duergar also brought with them ancient, lost smithing techniques from the Underworld, including some not seen since the Age of Cogs. The dwarves thought that they'd retake Underhome and restore it to its former glory, and the Duergar were eager to help with that; in fact, some regular dwarves were transformed INTO Duergar, because that's the only way to survive long-term in the poisonous reaches of the Underworld for extended periods of time. The Duergar promulgated the worship of their own god, Laduguer.
The Dwarf King was even a Duergar for most of the age, by acclaim and probably treachery. How did a Duergar ascend to the Dwarven throne? Did a Dwarf King become a Duergar? Who was he and why did he do it? By acclaim, and probably by treachery. The kingship of the dwarves is complex and multifaceted. Sometimes it is passed down from father to son. Sometimes it is passed down from father to DAUGHTER; there have been Dwarf Queens, although that's rare. Sometimes the Dwarf King will name an heir, who is usually, but not always, a close blood relative. The various dwarven clans have the right to pick their own king, which is usually only invoked when there's an unclear succession or when a Dwarf King tries to name an heir who is COMPLETELY unsuitable. Early in the age, only about a century after the emergence of the Duergar, the then-current Dwarf King died of what appeared to be old age. He didn't have a clear heir; there had been a number of accidents, and in fact a short, brief clan-war that got two of the leading lights of the younger dwarven generation, both grand-nephews of the king, killed. Most of the relevant claimants were unwilling, unsuited for kingship, etc. So there was an election. The Duergar, who had been granted full clan status, took part, of course. Nobody actually thought they'd win... but dwarven society was mad for all things Duergar at the time, and their leader was a real dwarf's dwarf, the kind of smith-warrior that is basically seen as the dwarven ideal. And a lot of the other clans deadlocked, so... In hindsight, many hundreds of years later when the true nature of the Duergar emerged... well, this all seemed MIGHTY suspicious. But that's how it happened.
In the Elven lands, the Shadow Elves began to delve ever-more deeply into the Underworld, in large part because the lore they'd shared to rebuild Concord had gotten them a lot of power and prestige among ground. They even created a new Icon for themselves, or maybe IT found THEM. But the Spider Queen granted the Drow immense power over the Underworld and its inhabitants, and blessed them with magics and mind powers that made them strong within her domain. She was even welcomed as a spiritual half-sister in the court of the Elf Queen, in the same way the High Druid was, although both of them instantly and cordially despised each other.
And in the Necropolis... The Illiches and the Lich King finally came to an accord with each other, with the Alhoon entering his service. The Alhoon had a lot of knowledge of and access to the Underworld, although the Duergars new and rapid expansion was constraining them somewhat. But it became much easier for the undead to make their way across the world below ground; they even discovered a technique wherein they could slay living dungeons and "raise" them as lich dungeons, slaves to the will of the Lich King.
The Duergar re-invigorated trade and industry throughout the Empire, with underways similar to the ancient passage between Forge and Anvil being established between all major dwarfholds and most of the major imperial cities. The Shadow Elves and the Spider-Queen brought forth much lore both the Red and Black Colleges were interested in, and the Spider Queen was consulted by more than one Archmage. A lot of people thought that the Empire, the Elves, and the Dwarves would gear up for a grand crusade to drive out the worst evil plaguing the Underworld and restore it to the storied glories of the first and second ages. That didn't happen... as we'll find out at the end of the age!
The Mind Flayers were taking advantage of the complete disarray of the Imperial Witch Hunters to do a whole lot of Mind Flaying, although the dwarves and the Shadow Elves were (supposedly) constantly warring on them to keep them in check. But there was a TON of contact with the Underworld during this age, and that meant lots of chances for the Mind Flayers to burrow up from below and start their weird cults. Only the Dream Police were organized enough to oppose them, but the Dream Police were regarded as something of a joke, a ridiculous relic of a bygone age whose secret police duties had largely been absorbed by the Hidden Hand. But they had a lot of psionic skill and were paranoid by nature, which gave them a BIT of an edge against the Mind Flayers.
Why was this a dark time for the Witch Hunters? Well, you see, for some reason, the populace was not so accepting of the fact that corrupted elements of the Witchhunters has pretty much wrecked Empire by unleashing super-powered Demidemons who proceeded to try and carve it up into their own fiefdoms. Between their supernatual beguiling, brute force, and demonicly enhanced cunning and trickery, it was short order before the clashes of the Demidemons basically resembled a civil war. And their attempts to cover up their own involvement didn't help persuade anyone of their shining nature, after a full age of basically bullying many of the other Imperial institutions. So the citizens of the Empire and the other organizations instituted what amounts to a reverse witch hunt, which more than decimated the ranks of the Witch Hunters, and wreaked many of their seats of power. They spent hundreds of years rebuilding their ranks, their powers, and their reputation. And even by the end of the Age it was only a sort of grudging acceptance.
The fighting was most intense immediately surrounding Axis, though through the efforts of the Legions and the Dragon Riders in particular, Axis itself never fell to any Demidemon. However nearly all the surrounding countryside, as far Horizon and Glitterhaegen saw some degree of conflict. There were also substantial conflicts in and around both New Port and Santa Cora, though as with Axis, Santa Cora proper was never violated by any Demidemon forces; the clerics of the city repelled any influenced forces that approached.
The most feared of the Demidemons was Seighardt Baumeister, who was an Inquisitor-Captain, one of the first cadre of those implanted with demon flesh in order to become powerful enough to directly fight greater demons. He was also the only one of the dozen to survive the process, making him the eldest and most experienced of the bunch. Physically and magically powerful, he was also obsessive and short sighted, and hated the thought of anyone having any sort of control or power over him, especially after his "ascension" and his belief that other mortals were simply below him. It was his prodding and manipulation that eventually led to the Demidemons deciding they no longer needed to hide within the Witch Hunters and kicking off the strife that signaled the end of the 10th Age. With his power, experience, and demonic tainted nature, it was easy to for him to quickly gather a personal warband of lesser Demidemons, cultists, lesser free demons, and greedy or corrupt mercenaries.
He was also the one responsible for the First Demidemon Siege of Santa Cora, as the thought of actual righteous god-botherers who had any measure of protection and counter to his power was an anathema to him. The priests and priestess that controlled the city, however, were just as cunning, and baited him into a trap that stripped him of a great degree of his strength and nearly destroyed his whole army. Eventually he was hunted down by a sect of Paladins with help from some of the few remaining loyalist Red College wizards.
The Demidemons largely preoccupied the Witch Hunters for obvious reasons; what did they do about the influence of the Duergar and the Spider Queen in this era? Very little. The central power of the Inquisition was largely wrecked for the majority of the age. Branch and local offices did what they could to fight off incursions of Mind Flayers from below, but especially early on, they were met with as much or more suspicion than the cultists and horrors that they were trying to fight. Later in the age, as they slowly recovered, they had suspicions, most especially about the Spider Queen, but their political capital was too low to do more than whisper suggestions that all might not be right.
In the 11th age, disgruntled Imperials, fearing the fall of the Empire, decided that the best idea possible was to RAISE THE ORC KING FROM THE DEAD. Given he was missing, not dead, this might not have been the best idea, but they were desperate. It remains unclear how these people got their hands on the rite they used to do this; there is some evidence that this had been attempted before by other groups without success. In this case, a group of Adventurers thwarted this at the cost of their lives. Everything seemed to be okay, but enough power was raised, it had to go somewhere...
That somewhere was a Goblin leader, who became the GOBLIN KING. Seeing the Elves and Dwarves were 'out of the way', or so he imagined, he decided it was time to CONQUER THE EMPIRE. It's also possible that the ritual power compelled him to try to take out the Empire. Ironically, he cleared Demidemons out of part of the Empire, but in the process, his army was chewed up. Nonetheless, he took over the Fangs and High Point for sixty years and even built GOBLIN CITY, his capital. The Elves, however, contained his eastern push and goblins vs. dragons went... badly around Axis.
Burgur-ghaz was a cunning king, and being elevated gave him more combat power, but he remained a goblin - successful kings must be strong enough to survive battle and smart enough to avoid unwinnable battles. He seems to have been driven to try to conquer the Empire by his transformation... but smart enough to know when to stop and prepare a better plan.
Goblin City was built on high ground on the coast of the Fangs, using Serpent Folk items dug up out of Throne Point. Burgur-Ghaz tried to drain the Fangs, but goblins were not well suited for it and slaves didn't really know HOW to do that well, though some large scale rice production was traded to Axis and Horizon, who needed fuel, in return for magic and metal. Normally, neither would have traded with goblins, but in the Demidemon Crisis, they were desperate. By the end, Goblin City rather resembled a budget version of Axis, complete with gladiatorial arenas. And the goblin sport of 'Kick the Kobold' spread to the rest of the Emnpire, but substituting an inflated ball for a kobold, under the name 'Battleball'.
Goblin City also cut deals with the Sahuagin. Who provided pearls and coral and fish in return for metal and land meats, and probably some human or lizard folk sacrifices.
The Goblin King tried to force other humanoids to obey him but he had limited success. Serpent Folk artifacts enabled him to command Lizard Folk and the kobolds bowed to him, but stronger humanoids defied him and Gnolls resented the hell out of him and harrassed his men but lacked the strength to eat them all. The Giants laughed at him but his effort at revenge was not successful, save for killing some hill giants. He skirmished with the Duergar but never tried to demand their submission. He never even tried with the Shadow Elves. Probably wisely.
Most were weak and ineffective and central authority decayed; they ruled little more than Axis.
No area was more devastated by the wars against and of the Demidemons than the southeast, the region east of Santa Cora. The collapse of Imperial Authority allowed forty-five incursions from the Iron Sea to break through and ravage the land. The cities of the area were gradually laid waste, and agriculture collapsed. It was not until the 5100s that the Crusader was able to push into the area and finally defeat the Demidemons. But the area was down to 10% of the population it had in the Tenth age and it would not recover in the twelfth, when the Wild Wood would overrun most of it.
Most noteworthy was Joachim the Tyrant (4883-4912), who briefly established hegemony over the area, only to then be killed by the monster known as Jelliblax, a giant pile of slime the size of the Imperial Palace.
The Barbarian State of Eld expanded into part of this area during the 5000s, but would be driven back to its current boundaries by the Crusader. Indeed, it is in this age that Eld comes together as a state, though a loose one.
It turns out the Duergar were... evil isn't entirely accurate, although they definitely are that. They're -skewed-. Most Duergar are quite, quite mad, and those who aren't are madness-adjacent. But it's the kind of madness that can go undetected for a long, long time. Their deity, Laduguer, is both dully engaged with the world, and is the sort of Chaotic Evil dumpster fire who wants to conquer the whole place and make his chosen people the rulers of everything.
The Spider Queen is more straightforward; she's just vicious and power-hungry and wanted to devour the Elf Queen and become ruler of all Elfkind, above and below ground, with her favored Drow lording it over everyone.
Laduguer, the Dwarf King, the Spider Queen, and a bunch of Illithid Elder Brains came together during a rare period of Underworld unity to launch an incredibly ambitious scheme; the complete venting of Underworld poisons into the atmosphere of the surface world. The humans would mostly die. EVERONE would mostly die, really. Those who survived would be twisted and corrupted into dark forms such as the Duergar were. The Duergar and the Drow had special rituals waiting that would transform wide swathes of their respective races into Duergar and Drow; everyone else they didn't really care about.
This took a long time to set up; all of those fancy underways and industrial infrastructure and magical sanctums the Drow and Duergar were building were in fact part of the vast network of poison fumaroles. They even struck a bargain with the Diabolist to vent Underworld poison through a bunch of Hellholes. None of them really knew precisely WHAT that would do but the Spider Queen and the diabolist thought it would be pretty neat to find out!
This might have worked, too, if it hadn't been for the Hidden Hand, the Dream Police, and what remained of the Witch Hunters. The Dream Police and the Witch Hunters had thought something was WRONG with the Spider Queen and that she had closer ties than she was letting on with the Illithids for quite some time. This was motivated by more than a little racism; those organizations were mostly human and humans tend to really not like Drow. They managed to sell the Archmage on a plan to capture and then 'decant' an Illithid Elder Brain, learning all its juiciest secrets.
This actually worked, ironically enough because Duergar-provided information made penetrating the Underworld a lot easier. Although they weren't prepared for the sheer depth and scale of the plan. The Emperor actually didn't believe his good friend and ally, the Duergar Dwarf King, could possibly be part of such a thing. And asked him to come to Axis so they could figure out who was framing him. The Dwarf King panicked and decided to try and set the whole thing off before they were ready. Many Underworld fumaroles in and around Forge vented; because of geography, the poisons rolled downslope across the Fangs and out over the ocean. This was the fall of Goblin City and the end of the Goblin Kingdoms, although the Goblin King himself would fight on until the end of the age.
What followed was an immensely destructive four-sided war. As the dwarves fought a civil war against the Duergar, the elves fought a civil war AND a race war against the Drow factions that had signed on with the Spider Queen, and the Empire and everyone else fought to find and eliminate the poison fumaroles before the denizens of the Underworld could bring them online. Accompanied by some human/dwarf and dwarf/elf infighting before everyone realized fully what was going on. This wasn't the kind of war that got resolved in one great big battle. It was close to twenty years of slow, grinding work; finding Underworld incursions, defeating them, rolling in with the Archmage and the Colleges to set down wards to stop them from recurring, rinse, and repeat.
Eventually the Duergar and the evil Drow were driven deep back into the Underworld. The Spider Queen was supposedly slain, but she's popped back up as an Icon once or twice, if for no other reason than ambitious, evil Drow see assuming her office as a path to power, or serving her as a path to same. Thankfully, the alliance between the Drow, Mind Flayers, and Duergar appears to have utterly unraveled; without an immediate goal their different flavors of evil just couldn't function in concert, especially since the Mind Flayers are... what they are. The Duergar are still around, and technically still are at war with the dwarves. They can be found aboveground sometimes, in places like Shadow Port. But they and the dwarves are essentially locked in a forever war that can only end with the complete racial extermination of one side or the other. This is made worse by the fact that there's Duergar blood in the main dwarf populations, because they ARE dwarves. This has caused... problems.
And that's how the age ended. The silver lining for the Witch Hunters is that they got a LOT of credit for being on the front lines of this whole thing, which helped them rebuild as an institution.