Arthurian Empire
436 - 455: Arthur the Great of the Britons
455 - 492: Mordred I of the Britons
492 - 501: Mordred II of the Britons
501 - 542: Mordred III of the Britons
542 - 579: Morgana the Great of the Britons
579 - 592: Slaine of the Britons
592 - 615: Guinevere II of the Britons
615 - 641: Mordred IV of the Britons
642 - 647: Guinevere II of the Britons
647 - 691: Tiffany of the Britons
691 - 734: Alaric of the Goths
734 - 742: Haakon of the Norse
Young Arthur was a formidable warlord, unifying Britain, driving back the Saxons, and forming an overseas empire that, after the 452 Conquest of Gaul, saw him march on Rome and despose Valentinian III. Unable to hold the Empire by force alone, Arthur formed the Round Table of Lords - where client kings would serve "as equals to each other, under protection of the Britons". The Kings of Gaul, Ireland, Iceland, Orkney, Daneland, and Great Rome met alongside the next in line for Arthur's throne, the Prince of Briton.
The series of bloody wars left him without an heir and his nephew Mordred became his heir, and even married his wife (a source of sharply suppressed gossip in the 450s).
Modred and his line would hold the Arthurian Empire against the Norseman, the Goths, and the Byzantinian Empire, though did not expand further: instead there was a series of great infrastructure developments to better manage the empire, bringing prosperity and, crucially, greater ability for Camelot to run the place. The Round Table members became increasingly discontent but a mix of divide-and-conquer tactics, financial bribes, marriages, and a few murders kept the peace - until a female monarch took the throne due to Prince Mordred's death from disease. Morgana was married to the second-born son of the King of Ireland but that, of course, meant nothing to the overseas kingdoms, who saw a chance to rebel.
Morgana turned out to be a highly capable and (from primary sources) quite sociopathic warlord, and the communication networks for the Knights of the Round Table were stronger than the rebels anticipated. The Salting of Rome in the second year of her reign ended any thought of rebellion (as well as seeing Great Rome hacked into two new kingdoms), and a bloody war of conquest saw the Vikings - who thought to take advantage of the civil war - brought to heel. Following this, there was peace and dominance of Europe for her reign and that of her romantically-named son and granddaughter. (The Visigoths, seeing how the wind was blowing, would join the Empire of their own free will for fear of being crushed)
The Empire would continue as a trading power and fight a series of brief wars against the Byzantiums but otherwise settle into a quiet stagnation, and the interconnected empire would see a primitive democracy develop among the landholding elites (including women, due to Morgana the Great establishing a precedent of matriarchy). Everything seemed stable - which was boring the hell out of Mordred IV, who considered himself an Arthur or Morgana in the making if he just got the chance.
In 626, the Persians and Slavs laid siege to Constantinople and Mordred saw his chance: he launched a war of conquest against the Byzantiums from the other side, seizing huge chunks of land. He had not expected this to mean "the Auld Enemy" would fall to the Persians, and soon he found his Empire in constant war against the Persians and Slavs both. Mordred IV was not an Arthur or Morgana in the making, forced into a humiliating peace with Persia and facing constant assaults from the emboldened Slavs and recurring Byzantine rebellions. Soon, discontent and small-scale rebellion spread throughout the Empire
The Empire needed a new great leader and unfortunately Guinevere II, while better than her father, was not good enough. The festering sore of Arthurian Byzantine was allowed to leave so the rest of the empire could be fixed up, but it was too late: various 'democratic' Althings had taken control of their regions, and they felt the monarchy was leading them to ruin. Camelot fell fast, betrayed from within by the queen's fourth daughter, and the vast empire became a republic: the monarch would now be an appointed leader with less powers, the servant kings & queens would be elected chiefs. (This took five years to complete, with the Siege of Iceland ending in a draw where the kingdom was allowed to remain)
The appointed Queen followed the advice of her Round Table and enjoyed a decadent lifestyle, while the newly republican empire saw a boom in localised control as various rich locals set up Althings and Round Tables of their own. A final decisive battle broke the Slavs but otherwise, the Empire focused inwards with the exception of the rich who relied on external trade. A new boom in wealth and showing off spread in the last years of Tiffany and in the term of her successor. However, under the uninterested eyes of the republican elite, even smaller Round Tables and Althings had formed among the common folk, or minority groups like the Jews, or among industries & trades left out at the big tables. And once the symbol of a monarch with a tie to Arthur of the Britons, there was no single thing to command everyone's loyalty, leadership was clearly Them Rich Fuckers.
It all came down when Alaric gave his blessing to a faction in the Round Table who wanted to explore and colonise the newly discovered "New World" of Greenland [OTL Canada]. Great riches were expected and would have been found, except first costs had to be spent on boats for a lenghty voyage, the establishment of settlements, and war against the various native Greenlanders who didn't know who these ghostly white people were but definitely didn't want them hitting them with swords. Discontent grew in the empire as the rich taxed the less rich, and every problem was blamed on "because the bosses only care about Greenland".
Haakon was, far as primary sources tell, a thicko. Not a bad person, just crap. He did what his Round Table said and he went to church. All fine if the Round Table had a grip - they did not. When the People's Revolts hit, the entire system shattered as every rich elite and region decided to look after themselves first. The Arthurian Empire collapsed into two dozen nations after the dust settled, with the Republic of Ireland (where rebellion had failed) as the single greatest power among them and the Greenlanders winning, learning how the ships worked, and turning up in Europe to trade.
(The Kingdom of Wales was formed around then by Arthur II, a self-proclaimed descendent of the imperial founder - and as far as we can tell, probably a real descendent of one of Tiffany's sons - and would feud with Ireland for influence in Britain until the Plague killed half of both nations and rendered it mute)