Where have all the Empires gone, long time passing

The nature of time is so deceptive.  A year is a long time and it is also so short. Just like a day. You get up and eat breakfast and go to work then you eat lunch, go back to work and then go home and do some other things then go to sleep and repeat.  But you write that report and you want it to end and it seems like it takes forever.  You sit down to watch a movie and the time passes  and you wonder where it went.  Days are like years parts of them last forever and other parts whisk by and 25-33% of them are spent sleeping and at the end of it nothing seems to have been accomplished.  After twenty years, though you reflect about how much has changed and you know that it all happened year by year and day by day.  Things change this way.  After a century you see the world has moved in giant leaps, well it has in modern society, in earlier millennia not so much.  But in millennia, things have changed as well.  More in our last century than any past millennia except one that includes our century.  

In Caranus it is the same, only it has been static for years, static change; no big changes in millennia.  But why?  There is magic.  Divine and arcane and there is not much difference between what everyone else has.  Magic is not something that changes substantially and it is more powerful than technological wonders.  Better steel, better forging techniques, different weapon variations are all trumped by giving an older design a +1 or better.  A better sword is less effective than a ordinary sword with magical powers; a katana made of steel folded fifty times, is sharper than a European Longsword, but with a +1 the latter will cut the former in half sooner in a battle.    So there is nothing to be gained by research except arcane research, so no one does it.  Magic is finite in power, but it stifles advancement and improvement.  

So why not a steady state of occupied nation states?  Natural disasters, wars, and magical creatures.  Natural disasters are things like volcanic eruptions, or volcanic explosions, massive storms, or floods.  They include pestilence too, but again spells and divine healing spells can cure a lot of these troubles, so what is left?  Magical plagues.  A plague that required a much higher level spell to cure would effectively make it incurable.   The town is struck down by a plague but the high cleric can only cure five people a day and after five days the entire populous is dead, save twenty-five people, only the incubation period of the plague is five days and it remains quite virulent through that time, so the plague might wipe out an entire nation or several, before a cure is found, and teleporting ahead of the illness spreads it further.  In theory, that would wipe out a large swath of nations, more than other natural disasters.  

Wars are less effective.  Most wars involve killing of the opposing soldiers only, when they are dead, the rest change hands and are in the conquerors lands.  Not much of a killer or impetus of change.  Wars happen all the time, they are usually small battles on the local scale, but sometimes they are bigger affairs.  Like on Earth where the Mongol Hoard conquered most of Asia and half of Europe in just a couple generations.  The greek and then the Roman empire, or when the Arabs conquered the remains of the roman empire in the late 7th century.  Or when the British conquered half the world.  There are plenty of examples.  In Caranus, the First Ones conquered the world, only to be reconquered by the Dragons.  The Orcish empire filled most of the continent before it was itself destroyed.  Since then there has been little of the fast growing empires.  Gutral Mastekeena took fifteen hundred years to get to its current size, and while large, it is yet to hold a twentieth of the continent.  It is big enough to withstand any large natural disaster, unless it is some magical plague, and it is big enough to with stand other dangers, but it is still possible to that some one might raise and army that might threaten it or that it might expand too quickly and collapse on itself.  

Magical creatures are threats.  There are creatures that can represent an extinction level event for a culture.  Mid-sized dragons represent a palpable threat to most small nation states, and certainly a town or small city.  Depending on the city, it might also end the nation.  Larger dragons would affect larger nations with similar effects.  The Free City of Dhewtudum, when it was attacked by the largest known Red Dragon, it cut off the Local Dwarven Empire from trade with human lands and others, the loss of trade nearly destroyed the empire because it was dependent on the trade to acquire food to feed its millions of dwarves.  One dragon, one city almost the end of an empire connected to other Dwarven empires that could have brought them all down and may still.  

There are other single creatures that represent a possible death to a nation, Mind-Flayers, Abeloth, Demons and things like powerful Undead.  The First Ones civilization came down during the war with the Dragons, but a few of their cities were destroyed from within when great mages became Liches and Demiliches and worse, Demises.  Liches are undead Wizards, when they decide to travel the worlds within Caranus they become a Demilich, but when they return with all their accumulated power and knowledge they become a near undefeatable threat called a Demise.  The first Demise was only defeated by a cabal of the most powerful wizards working together along with the remaining powerful dragons in a joint assault.  

The First Ones that live in Caranus are charged with the duty to fight most of these magical threats to the world, as part of the promise they made with the Greatest Dragons of the past eras.  They will seek out the unnatural magical threats from the creatures that are not allowed to roam Caranus free, the Mind Flayers, Aboleth, Demons, and powerful Undead.  They do not interfere with Dragons.  

In addition to the world sweeping natural disasters, there are the occasional invasions from the Wild Lands west of the Mirror Mountains.  Those at escape over the mountains and across the wastes either are individually powerful or are powerful in masses.  Sometimes these invaders become a naturalized resident of Caranus, like giants, gnolls, and others.  

So, where did the empire go that left these ruins?  How did they build these great magical treasures and then vanish?  Why is there wide forests here and a crumbled ruin with long buried roads and no connection to anything?  Why is there a ruined city here in the middle of this plain but no one living near by for hundreds of kilometers in any direction?  

Outside the sacked Free City of Dhewtudum, there are the ruins of three cities, Awsland, Galden, and Stuj.  Four ruined cities within a hundred kilometers that rose and fell in less than five hundred years.  All in the shadow of Gutral Mastekeena.