The Southern Empire didn't fall in a day, but its fall was remarkable sudden, dying in less than a century. One major cause of its destruction is detailed in the post on the Xannast Lords. The other major cause was a costly war, in which several key individuals were killed.

Understanding of those key individuals must come before the story itself. Two of these run very much together, detailed first...

Earth Defender and Juturith: These are the twin sons of a former emperor's second wife, taken in his elder days. They were not to inherit the crown, as the eldest son of the king's departed wife was before them in line for the throne. They did however, receive the highest education, respect, etc. that befits the sons of a king. They were both high-class wizards, indeed the last of the great wizards of old, though they were not the finest.
Earth Defender, which of course was not his birth name, earned his pseudonym by his love of the crafts of the earth. (His real name seems, embarrassingly enough, lost to history). The earth crafts were difficult to master, but he learned them better than any man has been known to.
Juturith came close to his brother in his knowledge of earthen lore, but used his knowledge for an altogether different bent...

*to be continued*

I love this line.

And then you end at the most interesting part...

Juturith came close to his brother in his knowledge of earthen lore, but used his knowledge for an altogether different bent...
The ways of the earth craft are strange. To understand how would take a lot of explaining, but let it suffice this way. Moving the earth is hard. It wants to stay where it was, how it fell, how it was created. Imagine the earth as a cloth. To change it's shape you either stretch and tear it, or fold it. In these terms, Juturith stretched, tore, cut, and burned the cloth, while Earth Defender folded. The whole still remained, unchanged in quality, merely re-arranged.
Juturith's heart was ever bent to himself, and while the polar-opposite could not be said of Earth Defender, his actions displayed a very different attitude. As the most prestigious of the wizards of the whole realm, these two were given jobs within the most prestigious branch of government: the military. However, the two could not work well in tandem, because of their different methods.
As their careers progressed, Earth Defender received the higher praise, as his pseudonym shows. Whole attacking armies were thwarted by canyons opening in their path. Enemy camps were sacked by his re-routing of rivers directly into their forts. Nearly always, Earth Defender returned the place to its natural state after the threat had passed. Juturith's successes were always raddled with side-effects, in no small part because of the earth's resistance to his ways.
At last, Juturith could handle it no longer. He went to the the Emperor, now his half-brother, and demanded leave to establish a wizard's kingdom. This term denotes the not uncommon practice of wizards establishing their own small realms, mining from the earth and building fortresses with their craft, often employing slaves or extorting labor from the surrounding countryside. Such kingdoms were generally ruled by rogues without scruples, and thus this request was strange, yet it was granted, though grudgingly.

Such kingdoms were generally ruled by rogues without scruples, and thus this request was strange, yet it was granted, though grudgingly...
Juturith at first seemed to break that mold of rogues, spending years hidden away building his castles and walls. His greatest masterpiece was a mountain unsurpassed in size. This mountain had existed before, but he heightened it double over, and then even brought it to life.
To us, it would be something like a science-fiction AI battleship, a whole fort that can think and even move many of its parts. While the mountain was immobile, it was also impregnable, sealing its doors shut at will. Indeed, his brother often ventured thither with him, delving mines together and building walls to the North and the East. Together they even ventured into Gorqua (meaning simply The Mountain) and cut passages. Their greatest work was the gem chamber, a small room among its many halls, but beautiful. Earth Defender wrought for his brother blue gems that glowed of their own accord, and these proliferated the chamber's walls, beaming or dimming at Juturith's command.
Yet for all of the good things that Juturith built, and how he expanded the Emperor's domain, suspicion remained. His words, whenever he returned to the capital, were few and curt. Ever they had, it seemed, another meaning hidden behind the long dark hair that covered his brows. None of the other lords enjoyed his presence, as it always seemed that he wished to make himself another of the great seven lords. But his intentions were both loftier and lower at the same time. . .

I would mean it as the element sort, but also as the dirt itself. The crafts of working with the dirt are strange because the dirt will itself resist being changed by craft. Check out the Red World Cobha page for a slightly more detailed definition.

But his intentions were both loftier and lower at the same time. . .
Juturith had a dark side, generally hidden from courtly life. However, even his own shady soul could not hide the fact that he was infatuated with the daughter of the Lord of Tomfie.
She was known among all the royalty and courts as the Daughter of the Realm. She was widely regarded as the most beautiful woman in all the South, though of course some pundits challenged this (but even they could not find a rival). She was also what we might call a tom-boy, to the point that she was the commander of all of Tomfie's armies and militia corps. She had made a rule, less than half in jest, that if any princeling were to want to marry her, he'd after to win in a duel to the death.
Juturith couldn't handle a sword, or any other melee weapon, to save his life. He usually stuck to lava fountains and instant graves as his main weapons of earth craft, with the occasional earthquake for drama. He'd have no chance at winning her that way. She in fact despised craft and wizardry, considering them the lowest form of warfare. Only for Earth Defender did she have any respect, and that only for his loyalty, not his craft.
But Juturith was not so easily cowed. What dark arts he used none to this day know, but it is certain that he cast some spell on her. She fell into despondency, loving only her warfare, forsaking the capita where she had so many loving friends and adoring fans. She took to wandering in the wilds with only a few of her most trusted warriors, killing dragons with swords (a nearly impossible feat).
She remained in that state for a year and a half, becoming quite the killing machine, but her father was not comforted by this. He had always hoped she'd marry into one of the other lords' families, or into the family of the Emperor. All her fans and friends were angry at the change, but she cared little for their feelings.
All blame was put on Juturith immediately, who had disappeared shortly after her mood had changed. Sure enough he was back in his mountain kingdom, mining more prolifically than ever with forced labor from the countryside. If he had any allies, they deserted him at that moment. There were rumors that he had spies throughout the whole realm, and that he used his magic and craft to learn all manner of secrets. All the Empire hated him, except his brother, who would not stoop so low as to hate.
At the next Council of the Lords with the Emperor, he was the top item on discussion. They invited Earth Defender to be present especially, although he was often present. At first the issue remained formal, investigating his labor practices, contacting and summoning astute wizards about his espionage, but then the Lord of Tomfie could control himself no more...

*to be continued*

but then the Lord of Tomfie could control himself no more...
Seething with rage the wrinkled man pounded his fist into the long table. "Enough inquiry, enough academics! Let us move to true investigation, let us seek charges against him!."
The Wizard of Thyron began to speak against this outburst, "Lord Arnah, we under..." but that was the last syllable anyone heard him say for a great while.
Lord Arnah, in an uncharacteristic disrespect, ignored the plea of the renowned wizard, and turning to the far end of the room, where Earth Defender sat with a hood over his eyes, away from the table, Arnah yelled, "Earth Defender, arise and explain this manner to us! You have spoke not a word while we wrangled over your brother for an hour. To noble are you to be suspected of any lie or treason," and while saying those words he looked at the Wizard of Thyron, "and we would never implicate you in any crimes of your brother's commission. Tell us what you know that we may decide quickly what it is that he has done and for what he can be charged!"

For a long moment and then another breath Earth Defender did not speak, or even move. At last he muttered, "Why do you call him my brother? He is no man of the South any longer for what he has done."

The lord Arnah relaxed his tone, and apologetically replied, "I am at fault for the outburst, and for putting you under such pressure. But if ever you would do service to the kingdom help us lay this issue to rest. Tell us what you know."

Earth Defender stood up slowly, his hood falling back off his head, "The Wizard-Lord Juturith, now resident in the East, is a greater potential menace to this empire than even the ancient foes in the North. His wizard-crimes, in the abuse of the earth, are manifold, but cannot be charged against him. No law exists to govern craft save honor, and he has none any longer, if ever he did.
"Among his chargeable crimes I will now divulge that I have long suspected him of necromancy and lemuelcy. While he employs and enslaves some living men from the surrounding country they have never ventured within the Ridge Ring, inside of which are his greatest mines. Even I have not seen these projects with my eye, but only by my looking at the earth and hearing its pleas. For these, he could be charged with expulsion from the Fellowship of Wizards, fined, and if he were ever to practice craft again, fined very heavily, perhaps even to forced labor. But these will not satisfy the wrath of Arnah, which Juturith deserves.
"I suspect him to also have concluded an alliance with the Black Cross, a secret deal by which he will attack us in the South if ever we strike against the North. And of course, knowing his ambition, he will strike in tandem with that ancient evil if he ever comes southward again. For this, he could be exiled, protected from death by his noble blood.
"Lastly, though, I suspect him of sorcery. As the court and the city have long suspected, I believe also that he has cast a spell upon the daughter of Lord Arnah. No punishment is written in the books for so great a crime, and so we I say it could be punished by death, for many lesser crimes receive the same. I suspect he has done this out of jealousy, knowing that he could never win her, no matter how much he desires her."
At Earth Defender's last phrase the face of Lord Arnah melted...

It was on that day, of the giving of leave, that Earth Defender spoke the first of his dooms...
The practice of doom-saying ended with Earth Defender. It is the strangest of the crafts, if even craft it is. Essentially when a doom is spoken, it cannot be altered. By one path or another it will occur. However, it oftentimes, in a strange way, depends on the saying of the doom. If the one who learns it keeps silent then the event may not come to pass, or yet it might.
Earth Defender spoke this doom on the day that his brother rode to the East to establish his new kingdom...