Of course, my plans to have a hero save Britain and then watch his descendants either continue his legacy, lost it or destroy it got derailed when my character, Arthur Cadarn, became immortal five years after I started. So as he united Britain under Welsh Catholic rule, the rest of the world went to dark ages from his perspective. The Umayyads took the Hispania peninsula (Arthur fought in the final campaign to aid the father of his daughter-in-law, but failed to prevent the inevitable defeat) and eastern North Africa coastline. The two Francia briefly united before collapsing into multiple civil wars as Charlemagne died early on. Pagans united and launched multiple great and lesser holy wars into Frankish lands, turning into a mismatch of pagan and Christian boardergore as the Catholics fought each other whilst falling to pagan invaders. Roman held on and strengthened their boarders, but made minimal expansion outside of the Balkans. Pagans (Norse, Slavic, Romuva, Tengri and those Finnish guys) reformed their religions and build up their Kingdoms (Norse - Sweden & Denmark, Slavic - Bohemia & Poland, Romuva - Lithunia, Finnish - Proto-Finland, Tengri - Large hordes in the steppes). Coptics suffered a resurgence as Armenian and Abyssinian empires formed and between them and the Romans, they manage to eliminate the Islamic rule in the Middle East with Zunist taking Persia and the Jomsviking in Oman (???). West African pagans formed the Kingdom of Mali while Italia managed to take Tunisia in a series of Holy Wars. With Catholicism falling and its authority diminished, many of their number converted to heresy.
All while this was going on in the 8th Century and early 9th Century, Arthur was uniting Britannia under his rule, giving tolerance to all Britons (Scottish, Irish, Pictish, Anglo-Saxon, Breton, Welsh) and bringing it into a golden age either as the rest of Catholicism was falling.
Then just as the day was dark, it got darker. The Umayyads invaded France, conquering most of it over the next century, conquering all remaining Catholics with the exception of two counts and driving back the Pagans. Italia fall under Fraticelli rule and over half a dozen civil wars, the Catholics slowly lost their power until they had been effectively driven from Italia and into Tunisia, where their power continued to wane. This ultimately resulted in them disposing of the Pope in Roma and installing their own Anti-Pope. In the steppes, the Tengri grew strong with the arrival of a Great Khan and while he was felled by his age before he reached Christian lands, the legacy hordes launched invasions of the Middle East and the Roman Empires. The Armenians swiftly lost land to the Pagans, breaking their military and causing the collapse of their empire as lesser nobles declared their independence and Muslims reclaimed some of their land. With a civil war breaking out as the hordes came, large swaths of Greece and Anatolia were lost to the pagans, ultimately resulting in a Khan in Greece claiming Constantinople and the crown of the Empire itself. The Abyssinian Empire, having conquered Egypt, Ethiopia and Yemen, collapsed into civil war with multiple breakaway states, including Muslims claiming their independence.
At the start of the 10th Century, Emperor Arthur Cadarn had been strengthen a united Britannia for over half a century and finally looked upon the rest of the world. Catholicism no longer ruled anywhere in mainland Europe apart two counts who held on due to nobody trying to seriously conquer them yet. Catholicism from had been driven completely from Hispania, was in the process of being wiped out in Francia, Germinia and Italia by heretics, infidels and pagans with the last bastions of it being slowly lost in Tunisia. The Pope himself had been disposed by a kingdom of heretics and was hiding out in northern Sweden. The rest of Catholicism fared little better with the Roman Empire having lost large amounts of lands and the throne to pagans whilst both Coptic Empires had shattered, slowly losing lands to pagans and Muslims. Meanwhile a new Great Khan was uniting another Great Horde in the furthest steppes.
Within century, there would be no more Catholics in mainland Europe and within another two, there would be no more outside of Britannia. Within a century, the Roman Empire would not just be a Christian empire ruled by pagans, but a pagan empire with Christian minorities. Within another century, no more Coptics would rule in the Middle East and within two, they would no longer live there.
Christianity was dying with only the British lands remaining strong as pagans, infidels and heretics claimed the rest of the world.
The next century would be known as the Age of Heroes as Emperor Arthur Cadarn put an end to the Dark Ages as they were peaking, with a hundred years of non-stop war as the British immortal restored Christianity to its rightful glory.
First he brought the Pope under the safety of his rules. Secondly, he had the Pope declare a Crusade against the heretics in Italia. With his standing army of over twelve thousand strong, he swiftly crushed the heretics for the lands of Italia had been bloodied by decades of civil war between heretics and faithful. Next he turned his attentions to the Muslims, readying another Crusade. However, Arthur's plans were waylaid as the Italian nobility placed a pagan warlord as king, much to the British warlord's bafflement. As of such, the 2nd Crusade was the liberation of Italia like the first. As he left in his ships for the second time, the Italian nobility elected the younger brother of the old pagan king as their new king. The 3rd Crusade was far more brutal than the first two as Arthur had grown fed up with the Italian nobles, claiming the kingdom as his own and clearing out the old nobility, placing loyal and faithful followers in their place. The 4th Crusade was against the Muslims in Hispania, striking at the rich western coast. Seeing that the war would be a large one, Arthur brought now only his standing armies, but his personal retinue with both armies totalling over thirty thousand in total. However, Arthur underestimated the sheer size of his foes and while he saw plenty of initial success, he was slowly losing men and women as he crushed armies of equal size while the Muslims were able to continue feed more and more soldiers into the meat grinder. Realising that he was going to lose from attrition, having lost almost half of his original army, Arthur brought in levies from his subjects in order to hold the line against the Muslim hordes. By the end of the 4th Crusade, over sixty thousand British warriors had fought in Hispania with half of them being lost there while the Muslims had committed over three times that many and lost over half that many. It was the most perilous point of Age of Heroes as while it broke his biggest threat beyond recovery, Arthur would have lost if the war had continued for another year or two and it would had been the Christianity which would have never recovered.
The 4th Crusade was a turning point as it broke the backbone of the Umayyads and Arthur never gave them the chance to recover. Launching the 5th Crusade against the largest pagan kingdom in Francia whilst declaring several lesser holy wars against the various pretty kingdoms and warlords in Francia, striking up through Italia and across all of coastlines, taking what land he could in order to strengthen his position before the Umayyads could recover. Within five years of the 4th Crusade ending, a quarter of Francia was under Arthur's rule. Arthur never stopped the wars, seeking more and more foes, declaring holy wars against those on his boarders in mainland Europe and seeking out disposed nobles with claims on lands that he could not reach with his holy wars. The 6th and 7th Crusades were declared upon the Umayyads, seizing large chucks of Hispania and further stripping them of their richest and most populous lands. Various British dukes and duchesses also decides to join in with the Age of Heroes, declaring their own wars to claim or reclaim land for the Britannia. By the halfway point of the 10th Century, all of Italia and Hispania were under Arthur's rule along with most of Francia and Germinia with only a few holdouts of Muslim and pagan resistance that Arthur couldn't be bother to put down whilst he had other things to worry about and his nobles were willing to the job for him. Holy Orders were founded with the biggest being the Knights Templar, the Knightly Hospitaller, the Teutonic Orders, the Knights of Santiago, the Knights of Calatrava and all of them having their Grandmasters answer to Emperor Arthur Cadarn whilst the Pope was restored to Roma.
With Catholicism secured, Arthur turned his gaze to eastern Christianity. He married one of the daughters of old Roman Emperor, using her claim to the Throne to dispose the pagan Roman Emperor and get rid of the elective monarchy that had been installed by a greedy nobility. He installed his wife as Empress of the Roman Empire using his armies and then aided in restoring the Roman Empire to its glory from two centuries ago. The new Empress was highly unpopular, but her British powerbase was untouchable and her rule secure as her opponents either met an assassin's knife in their back or a soldier's sword in their front. With the Roman Empire secure and set to become a loyal and valuable ally once it had recovered, Arthur turned his attentions to the Coptics as they fell to pagan and Muslim. Opening his coffers, he lent coin and soldier to their wars, giving them a much needed edge over their foes. Within a decade, the Armenian Empire was reforming while the Abyssinian Empire had mostly recovered.
While his eastern adventures were ongoing, Arthur's subjects had fought their own battles in the west with Tunisia expanding while Francia and Germenia were secured with pushes eastward and northward, the former stopped by the Slavic Kingdom of Bohemia and the latter by the Norsemen retaining control of the seas, halting any advances beyond the Danish peninsula. In the late 10th Century, Emperor Arthur Cadarn declared himself the Holy-Emperor of the Holy British Empire, having sought the approval of the Pope and the friendships of the Romans, Armenians and Abyssinians.
However, Arthur still had threats to face as the Great Khan had arrived at the borders of the Roman Empire by Crimea and Georgia. Whilst having mostly recovered, the Roman Empire was still too weak to fend for itself against another Great Horde. Unwilling to abandon his allies and let the Roman Empire fall to pagans once again, Arthur sent his personal retinue, numbering over thirty thousand now, to aid his wife's armies while rallying a massive army of over a hundred thousand men and women. Taking this massive army to Kalevan, where the Great Horde boarded the Baltic Sea, Arthur and his commanders craved their way through undefended lands as the Hordemen fought the Romans and British in the south. Eventually, the Great Khan mustered up a grand army to face Arthur. With each side having various advantages, Arthur and the Great Khan fought each other to a standstill, both losing tens of thousands of soldiers with the latter having it worse. An uneasy truce was made when the Great Khan died and the war for the Roman Empire with him, but it lasted a mere two years as Arthur soon marched on the Great Horde before it could recover from the last one, bringing over eighty thousand soldiers through Kalevan. The war lasted five years as Arthur sought not to take land, but to slay the pagan soldiers so that the military might of the Great Khan could be broken. With eight out of ten men dead by the end of the war, the Great Horde collapsed into in-fighting as the Great Khan was unable to hold onto his power.
Returning home just as the 11th Century, Arthur defeated the last major threat he would ever face and the Age of Heroes came to an end along with the last serious Christian expansions in the old world. Whilst the New World was colonised by the Holy British Empire, the Christian Empires barely expanded over the centuries and instead focused inwards while fending off the odd pagan invasion or raid.