Invasion of the Midlands and the complete collapse of the Suielman empire CR 150
It was invasions from the north that eventually put the nails in the coffin of the Suielman Empire. The first horde of humanoids swept through the Giantdowns in 75 CR, destroying the Suielman outposts in that region and even taking Metamor Valley itself. The Imperials rallied against the onslaught and drove the armies of lutins and giants back into the Giantdowns, but this came at great cost to the Empire both in funds and personnel. More invasions followed, some more destructive than others, and the Empire's ability to contain the hordes eroded. Finally, in 150 CR, the humanoids broke through the Valley and flooded into the Midlands, as a good portion of the Imperial army scattered and fled before them. Those who maintained their discipline in retreat fell back to the Sathmoran provinces, helping to evacuate as many humans as possible along the way. The emperor at the time: Thorius Falernus proven to be worse then useless and his bad handling made things worse. His assassination in 151 came as a surprise to no one. A person claiming to be his son (Gessius Pertilius) took power in Elvquelin and proved to be the right leader for the task. He is credited with rallying what remained of the military in Sathmore that was able to hold off the invaders but at great cost in lives.
In Pyralis Laetonius Pertilius (the younger brother of Gessius) finalized the split of the two halves of the empire by declaring himself High Lord of Pyralis. Busy trying to stop the invasion Gessius was in no position to argue the point. The two now separate empires had all they could to simply survive.
Both were greatly helped by the terrain especially the Sathmore Mountain range. It divided the two regions and forced the invaders to go around it. At it's northern end the range comes close to the Sea of Stars leaving a navigable gap of only a few hundred miles. This allowed Gessius to concentrate his troops in a small area. Even so the fighting very very bloody as the Sathmorans fought with the ferocity of a mother bear in her den, while the horde's ambitions were depleted by their already-substantial winnings. Furthermore, the giants were hampered by their inability to survive at temperatures much higher than 80 degrees Fahrenheit; as summer came into its strength, they were forced to abandon their conquests in search of cooler latitudes. The lutins were limited in what they could accomplish without the giants backing them up, and the assault was stopped at Sathmore's edge. The cost in human lives was tremendous. One unit whose records still survive (the 3rd legio) had 8,250 men listed on the roster in cr149. A roster from cr151 reported only 112 of those original legionnaires still alive.
To get to Pyralis the hordes had to go through the Southern Midlands, around Godswood and then across the Flatlands. Even so Pyralis was hit hard by wave after wave of invader. The fighting was less organized thean in Sathmore but every bit as vicious and bloody. Huge swaths of territory were devastated. Twice the attackers reached as far south as Pyralis city itself but very strong walls and brave defenders helped save the city. Even so 75% of the city was destroyed. The death toll for the whole region was so high that population levels didn't return to their cr149 levels till 520.
There was no official end to the invasion. The attacks just tapered off in 151-152 as more and more lutins retreated back to the Giantdowns or were killed. Gessius declared the invasion over on June 5, 152. Laetonius did the same in March 12, 153 but both continued to suffer scattered raids for several more years.
When the invasion was finally beaten off both countries were exhausted and worn out and the entire Midlands was in ruins. Sathmore and Pyralis had survived but the empire hadn't. Neither was in any shape to rebuild the empire.
The death of Thorius Falernus and the rise of Gessius & Laetonius Pertilius is universally considered to mark the end of the Suielman empire. The Sathmore empire rulers continued to call themselves Suielman emperor for a long time afterward (and some still do) but the empire was truly dead.
To this day June 5 in Sathmore and March 12 in Pyralis are holidays celebrating the end of the invasion.