The collapse of The Alammar Empire was a complex multi-faceted process taking over hundreds of years. The overall age of the Empire began to prove a detriment, as the age-old institution began to age, unable to keep up with the changing times. This inability of flexibility was due to a bloat in the imperial bureaucracy. While necessary to run the country, it had grown to such a degree that a majority of its committees and organizations were entirely obsolete, working with numbers that were simply not real. It had grown like a plant that hadn't been cut for years and years, but had involved itself within the Imperial Court and the politics of the Empire for so long that reform was almost entirely possible, not without great effort. The size of the bureaucracy compared to the needed size of administration was needlessly overweight, and served only to gobble up funds and resources better used elsewhere.
The High Kings, who were supposed to rule the Empire unilaterally and keep all the institutions of the Empire in check, increasingly failed to do so, withdrawing both from political and public life to focus more on private pursuits utilizing their vast wealth and power, squandering public faith. Their faith in the Alammar Empire to continue existence without their intervention only grew with each successor. The Empire was, in their eyes, too big and too old to fail. The work of their forebears had cemented a golden age for the lot of them, and thus they only needed to reap the rewards, seeing life as changing and static. These private pursuits and rejection of governance only served as the cause for countless scandals within the Empire decreased the already shortening public faith.
Laws within the Alammar Empire, which once seemed so sure and strong were now crumbling under the test of time. Countless legal codes, each contradicting each other, refuted attempts at organization, leading to headaches for judges and legal students, who now elected which laws to follow and which to ignore, causing a Wild West of legality, where anything went so long as the judge saw it that way, leading to a heavy increase in judicial power in the outskirts of the Empire.
The chain of command was also beginning to break down, with further authority and power given to Provincial Governors in order to establish order in their respective corners of the Empire. As such, they grew more and more independent, and more able to simply ignore the demands of the High King now that power no longer flowed downward from the crown, though they often did so to preserve the veneer of a unified Alammar Empire, which benefited them as well. In reality, however, each province was beginning to develop its own independent system of governance and keep a share of the taxes, ignoring or marginalizing imperial advisors and bureaucrats.
Landowners also grew more powerful. A long time has passed since the time of High King Alfred, and thus the age old restrictions that had been put in place to limit their power were either too old to keep their effectiveness or simply weren't enforced by the bureaucrats and other officials in charge of doing so, causing another boom in landowner power. Wealthy merchants or those from rich families would buy cheap land out in the country from the poor (or otherwise bully it out of them) and build their own little legal empire of land. They would make this land worth its cost by getting these poor farmers to work it for very little while producing crops they could sell for a fortune.
While in theory this too partially benefited the state with increased agriculture and all, most of the time these landowners were looking for either money or political power. In the case of money, they would grow almost exclusively cash crops like tobacco, which didn't directly help the Empire in any way. For political power, the influx of agriculture usually came with the added provision that this would grant the landowner power, and lead to the formation of a Landowner Bloc within the veins of imperial systems and institutions, hindering further reform or growth. This influx of agriculture was mostly fake- the formation of landowners barely increased the production of necessary crops.
The Imperial Army, the major force meant to keep the Empire together and to defend it in times of crisis was extremely weakened. For one, it had barely seen a major war against a peer enemy for millennia. Its doctrines and tactics were ancient and newer strategies were rejected by traditionalists within the military. It had failed to adopt new technologies like iron weapons and armor, and still struggled with a chariot-less cavalry force. Even worse was the dividing of the Imperial Army. In the days of William the First, the Imperial Army was a roving band of trained troops that simply went along and masterfully defeated its enemies. Following the reforms of High King Alfred, the army was split into provincial garrisons as a police force to be kept on standby in case of a crisis. While slightly mobilized during the Pacification Wars, they almost never saw widespread action and full mobilization. Those that volunteered to join the army mostly did nothing except perhaps fight against groups of bandits in the countryside, meaning the ground troops were almost entirely green and inexperienced.
The garrison method, while working to demobilize the army, also had the unintended side effect of causing more local sympathies than national ones. While Alfred had planned on his successors building a strong, shared national identity, they hadn't done so to a satisfactory point. On average, the local soldiers probably held more local alliances to their villages and communities than to the Empire, meaning that popular revolts and rebellions would be hard to put down. The main modern purpose of the Imperial Army at this point was purely as a deterrent from any serious rebellions than an effective fighting force.
The mounting military and political weakness of the Alammar Empire was not the only reason for its downfall. Things like famines from improper distribution of food, changing climates and blights caused shortages of food, causing many to starve and a massive outrage when it was revealed that High King Victor and other powerful members of the Empire were feasting like kings within the walls of Ajandah while peasants and other undesirables were starving to death right outside. Not only that, but new diseases caused by magical meddling with medical practices swept throughout the Empire, killing tens of thousands.
Culturally, the Alammar Empire was also stagnating. While in the early years of the Empire the arts had exploded in popularity with the New Revolution movement, literature and art stagnated during the middle years and just stopped during the end years of the Empire. Without new inspiration, the great works caused by the start of the Alammar Empire were now being strangled to death by it, causing endless repeats of the same plays and marveling at the same few pieces of art without innovation.
The Knights of Solamina, the Alammar Empire's premier special forces and enforcers of peace were on their last legs. Reduced to just four individuals, they were unable to keep the peace everywhere and were the last of their line. Some within their ranks even held sympathies for the local populations, compromising their effectiveness.
Popular revolts also began to pop up. Though sometimes limited in scale, they did serve to weaken the authority of the Alammar Empire, which was already severely depleted, and to debase the deterrent threat of the Imperial Army through their incompetent reactions to these revolts, and to serve as a prelude to the eventual Splintering.