Third Astral Era
In the wake of the Third Umbral Era and the Calamity of Fire, the people of Eorzea were disenfranchised with the gods, having lost faith in them despite bringing them to the center of their society. The communities that arose from the ashes of the Second Astral Era decided to focus on the growth of the individual and the community, and began making strides in both technology and magic, advancing civilization at a rate unseen before. The man that history holds almost entirely responsible for this unprecedented growth; Xande.
Birth of a Nation
Xande rose as a leader in the early parts of the Third Astral Era, and united both the people who sought to advance technology, and those who were descendant from priests and mages of the Second Astral Era, inviting both to constantly push their disciplines, and eventually finding a method to combine the two, dubbed Aetherochemistry. This advancement launched the people of Allag further into the future than anything else before. Technology was greatly enhanced, from transportation and societal needs, to warfare and siege engines that could fire loads much heavier than their maximum weight capacity much further than their maximum distance. This, on top of the hand-picked mage cadres in Xande's army, none could oppose the might of the newly born Allagan Empire.
Xande and his armies crushed any and all who opposed him. He soon conquered all three of the Three Great Continents, hanging the imperial standard in every city. With no remaining opposition, a heretofore unimaginable peace swept through every land. Xande spent his remaining years carefully grooming his successors so his hard work would not fall to ruin with his death. At this point, the Empire began focusing on further improving society and general living rather than warfare. One such project was the construction of several crystal spires that gathered energy from the sun to fuel its technology. Finally, Xande was interred in a crystal cavern near the area that would become Mor Dhona due to the large amount of crystals there that the Allagans believed would channel the pure Aether and heal the mortal body, should they ever find a way to undo death.
Beginning of the Decline
For over three hundred years, there was peace, with no war or bloodshed in the kingdom. But eventually the once proud kingdom fell to indolence. With their lives mostly automated, the need for innovation waned, and the people grew complacent. The kingdom was slowly decaying from the inside out, with birthrates dropping and suicide becoming an ever increasing event, and leaders became more corrupt, causing small uprisings.
Eventually, one man sought to solve this problem. Amon was a technologist considered brilliant in his time. Believing that the people needed a leader to inspire them, but knowing none even amongst the royal family had the charisma and power to do so, he began setting about his greatest work: finding the path to immortality and resurrection. He began experimenting on rebels and insurgents, such as Glasya Labolas and General Phlegethon, or political rivals who he promised untold power to, like Scylla. He experimented on both the royal line and himself, creating several clones of both Unei and Doga, the current children of the Emperor, until eventually he felt confident enough to unearth Xande's tomb and finally begin working on bringing the once great leader back. In due time, he was successful, and Xande walked amongst the living once again.
The Empire Reborn
Amon briefed Xande on what had happened to his once great empire, and he set to work correcting the mistakes of the past. He had the current emperor assassinated, as well as any of his sympathizers and revolutionists. He then rebuilt his once glorious army back to its former strength, and set his sights on their newest conquest: the home of the Dragons to the south: Meracydia.
Xande had once again ignited the people's hearts, spurring inventions and improvements to their siege weapons and equipment. The dawn of their conquest to the south began, with their first initial attacks generally favorable, pushing the Meracydians back. However, they soon came up against the Dragons themselves, who made quick work of the Chimeras of the Empire, driving them back to the beaches once more. The battle waged back and forth, with the Allagan scientists pushing their Chimerobiology and Aetherochemistry further than ever before, eventually creating horrific beasts that could not be stopped. In a bloody and decisive battle, the Empire managed to slay the great Bahamut, pushing further and further into the land.
The War against Meracydia
In a final attempt to survive, the Meracydians evoked their gods in the first ever summoning rituals recorded (this is believed to be under the influence of the Ascians), calling forth Sephirot the Fiend, Sophia the Goddess, and Zurvan the Demon. These beings became known as The Warring Triad, and they drove the Allagans back as far as the shores of their land. Even if the Empire managed to fell the beasts, they would simply appear once again on the battlefield.
With the war effort being actively rebuffed, Xande grew furious, demanding his mages find a solution or he would kill one of them a day. Eventually, the Empire formulated a method of neutralizing the summoned gods and imprisoning them. This branch of magic would be the foundations of modern Summoning. This advantage only lasted for a short time, however, as the wyrm Tiamat, in her grief, summoned Bahamut back once more. The twisted shade that returned scorched the earth, driving the Empire back once again and robbing them of what little advantage they had.
The bloody war came to a climax when Xande, eager to prove his worth by slaying his new foes, entered into a pact with the Voidsent Cloud of Darkness, allowing her minions to take over the corpses from the war to serve in his army, in exchange for allowing her and her kind into their realm to feast on their aether. This drastically turned the tide of the war, with each dead enemy or soldier being reanimated and added to the Empire's ranks. Eventually even Bahamut was not enough, and the king wyrm was captured and collared, used to power the rest of the campaign. Meracydia fell soon after.
After the War
With Meracydia captured and Bahamut imprisoned in Neurolinks, the Empire seemed to be in a stronger position than ever before. However, with nothing left to fuel him, Xande began to feel the gnawing presence of death once again, slowly growing mad. He sought to bring an end to all life, so that all may experience what he yearned for. He had Bahamut imprisoned in a massive satellite, and sent into orbit to directly absorb the energy from the sun, with the understanding that the great wyrm's fire-aspected aether would gather more energy than any of the towers that covered the land ever could. He had the Crystal Tower fortified then set his scientists to creating a way to open a portal to the Void, so the Cloud of Darkness could come through and rain death upon the world.
Beginning of the End
Many rose in opposition to this plan, including the cloned royals, plunging the kingdom into civil war. Many fought to reach Xande and stop him before the portal could be completed, but Xande activated the satellite remotely, beaming the accumulated energy to the tower directly to power the portal. However, the sheer amount of power caused the very earth beneath the tower to crumble. This set off a chain of several destructive earthquakes, destroying the entire realm. Before the kingdom fell completely into ruin, Amon used powerful magicks to seal the Tower and everyone within in time, placing the tower and its inhabitants into a deep slumber.
When the earth finally settled, the kingdom had been almost entirely buried. Princess Salina, the last surviving Allagan royal, survived. Knowing what Amon and Xande had done in the Tower's final moments, she passed on the gift of her blood to another, a Miqo'te named Desch, to ensure that in the future, when the Tower rose once again, there would be someone who could unseal its gates and stop the horrors within. The clones also hid away from the world within the tower, biding their time until a chosen hero could defeat the last remaining nightmares the Allagan Empire had wrought. The few people who survived had completely lost their ability to use any and all of the technological advancements they had come to rely on, leading to a dark age the likes of which had not been seen for several millennia. This was the beginning of the Fourth Umbral Era.