Under This Sign: WIP
Here's something that ive been working on for a while.
Main POD: The solar event that occurred before the battle of the Milvian Bridge does not appear to be a cross, but instead a Great Sunburst, the symbol of Sol Invictus. As a result, Constantine conquers under this sign instead. He does not issue the Edict of Milan and the Christian persecutions continue. He also holds the Council of Ravenna to sort out the large number of continuing religious and social problems the Empire was having. The largest result of this Council was the declaration of Sol Invictus as the "One True God of all the Earth", effectively throwing out the Old Gods, such as Jupiter, Venus, Mars, etc., in favor of a centralized version of the Imperial Cult. However this did not solve the growing "Christian Problem" of the Eastern and African provinces, especially in Carthage, Antioch, Byzantium and Athens. This would continue after Constantine's death in 330 from heart failure.
By 380, the two halves of the Empire had grown decidedly apart. In the West, the many, many improvements, fortifications and rebuilt areas of the provinces done by Constantine the Great and his successors had managed to stabilize that half of the Empire. The armies were populated by mostly Romans or Romanized Thrungians (Germans), Dacians and even a few Goths. The taxes remained strong, if significantly lower than in the East, and the people were devoutly worshiping Sol Invictus (The Unconquerable Sun) as their God, and the Emperor as the arbiter of his Divine will. The East on the other hand was not nearly as stable. The lack of a stabilizing force (what Constantine's reforms would provide IOTL) was beginning to pull the different cultures and peoples of the East apart. However, there were two major forces that were holding the Eastern Empire together, Christianity, the common religion, and the Sassanian Persians, the common enemy of the Empire. The large number of trade routes that ran through the eastern Empire also provided a LOT of money, which didn't hurt matters. It was this cash that allowed for the funding of new recruits and mercenaries alike, which not only helped keep the Empire stable, but also helped keep it from being invaded by the Persian, and the "Western Heathens".
No one in the East knew that the end would come from, what they thought of at least, as a bunch of ragged and smelly northern barbarians. In 385 the Goths (the entire tribe, not just the Visigoths) arrived on the north bank of the River Danube, seeking refuge from the Huns and other steppe invaders. The Eastern Emperor allowed the Goths to settle in the north areas of Pannonia in exchange for converting to Christianity and supplying the Eastern Romans with tribute and troops to fight their greater enemies, the Persians. However, the Romans treated the Goths horribly. They thought of them as outsiders, scum, and simple minded barbarians. The addition of an outbreak of famine among the residents of the area (probably caused by the Goths arrival) forced the Roman Emperor to give them horrible food and water for several years. Many families were forced to sell their children to the Roman army to feed themselves. After the famines subsided, the Eastern Romans began to use the Goths as fodder in the army, giving them horrible weapons, armor and training and shoving them into a battle line to take the largest brunt of the assault so the Greeks, Egyptians and other people of the Eastern Empire that had reached acceptance with Roman society did not have to.
After about 10 years of such treatment, the Goths revolted. In 395-396 many of the Gothic tribes stopped sending the Romans their tithes and troops, in order to gather theses supplies themselves and prepare for war. The Eastern Emperor Valens took this as an affront. He saw it as ungrateful barbarians rebelling against the hand that was feeding protecting and supporting them, just to gain power. However, he was preoccupied by the Persians and so sent several of his commanders to confront the ingrates and upstarts. Several minor skirmishes followed, all of them Roman success of course, but none able to pin down the rebels. After a year or so, Valens had managed to subdue the current Persian attacks and was freed from duty on that front. So he turned his attention to the Goths, intending to finally subdue and break them. However, over the course of the year the Goths had managed to break into numerous Roman armories and storehouses, greatly increasing their arms and effectiveness. In 397 the Emperor confronted the Goths, highly confident of his success against a group of unwashed, poorly armed and badly supplies barbarians. north of Adrianople the Goths proved otherwise. The battle was almost a one-sided massacre of the Eastern Roman army. The Emperor Valens, whom otherwise might have gone on to be called "The Great", was killed, as was much of his court, brought along to witness his power over the Empire by crushing the rebels. The battle was an unmitigated disaster for the Eastern Empire, the dynastic feud created by the death of Valens and a LARGE number of his potential successors, completely destabilized the Eastern Empire at the worst possible time.
By 418 the Eastern Roman Empire had fragmented into a number of pieces, the largest of which was held by the Visigoths, whom restyled themselves as the Vanarii, a throw back to the old Roman times. Other tribes that had settled in the east were the Burundians, whom settled in Achaea and rule through a series of puppet Emperors descended from Valens, the Lombardi in Tarsos, the Alans in and around Antioch, and the last true "descendants" of Valens having stayed in control of Aegyptus and Phonecia.
The western Thrungians were not far behind picking up on the lessons of Adrianople either. However, the trouble started for them even before the Battle of Adrianople. Feuding between the Visigoths and Ostrogoths was flaring up before the battle due to the lack of success was reaching a head. However, before the break could be completed the Eperor was killed at the battle. Both tribes claimed they had been the one to kill him. This finalized the break. It was the Visigoths whom reached the Sea of Marmara first and claimed its riches. The Ostrogoths were forced west, first to Greece which paid them with plenty of tribute money to not kill them, and then north towards the Balkans, which brought them into contact with Western Imperial forces. While the Ostrogoths gave the Romans a very large run for their money and wrecked much of the Balkans, they never decisively defeated the Western Romans, like they had done at Adrianople. and thus were forced to migrate elsewhere. They finally wound up in Bohemia, forcing the Quads and Macromman, whose had been fighting over territory at the time to flee westwards. Bohemia was often described as a natural fortress, and the Ostrogoth also made major improvements to its defenses to keep out the steppe peoples, especially the Huns.
The fleeing Thrungians coalesced with the Vandals and Suebi, other tribes fleeing the advances of both the Ostrogoths and the Huns. In 404 they reached the river Rhine and tried several times to cross it without success, both due to a lack of transport and the Roman garrisons fending them off. With other tribes closing in on them, it was a miracle that they managed to succeed at the crossing in the winter of 406. On December 21, 406 the upper layers of the river Rhine froze solid, allowing many of the Thrungians and Vandals to cross unimpeded. This was a disaster for the surprised Roman garrison. While the other garrisons along the river responded quickly to the hole, by the time it had been sealed, and that required bringing in the entire Britannic garrison, much of which was wiped out by the following Thrungians, leading to the loss of the province in 429 to the Saxons, Jutes and Angles, by that time the Vandals and their allies had either moved on or been wiped out. After a brief attempt to settle in Aquitania, the Vandals were allowed to settle in the troublesome province of Baetica in 413, one of the last places in the Western Empire with a significant Christian population. It was here that the Vandals were converted to Christianity by said population.
In Britannia, the leaving of the Roman garrison caused massive problems. From the north the Picts invaded, internal squabbles long suppressed flared anew and from across the North Sea came the Saxons, Jutes and Angles. At first they were used as mercenaries to destroy the Picts and help keep order in the province since the Romans could not send a replacement garrison due to contuined Thrungian incursions against the Rhine frontiers. While it worked well enough for the moment the peace was not to last. In 425 bad winters and poor harvests led to a great famine across much of northern Europe. In Britannia, this made the Britons unable to pay the Saxons their annual tribute. The Saxons responded with violence, overthrowing many of the Briton states. This allowed other Saxon tribes to migrate from Jutland, which was being invaded by the Dan, a Geatic people from Scandinavia whom were being forced out of their homeland by other Geatic tribes due to the famine. By 429 most of the province had been overrun. While some of the Britons, mostly in Wales and Cornwall, managed to hold on, the Romans would never reestablish Britannia as a Roman province.
Back in Europa, the Romans had successfully repelled a large number of the Thrungian tribes, with the only major tribe to settle in Roman lands being the Vandals and Saxons. The final major test of this period was, rather inevitably, the Hunnic invasions. In the 450's, the Huns overran large portions of Eastern Europe, with the majority settling east of Bohemia. While the Ostrogoths managed to only have to pay tribe to the Huns, the other Thrungian tribes were not so lucky. The Huns managed to carve out an Empire of vassals and conquered people for themselves by 458. It was then that they turned their attentions to Roman lands.
In 460, a Roman princess by the name of Valoria, was caught up in a scandal that was in reality had nothing to do with her, but Imperial politics forced her family to have her banished to serve as a keeper of the Solar Flame in some distant province. Not wanting this kind of a fate, she had a message sent out to the king of the Huns, a man named Clothodus promising her hand in marriage and rule of the Empire to him if he could rescue her from her prison. Clothodus, probably the greatest Hunnic king, immedialty saw the potential for profit and power and set out to "rescue" the foolish princess. In 461, Clothodus's massive army attempted to cross the Rhine when it was at its lowest. He was met by several Roman generals whom almost managed to stop him. However, ata key moment in the battle the lead general was killed by a stray arrow, and his army broke and fled. While Clothodus could not take advantage of this turn of events, he did have access to the lands of the Empire now, and was pressing hard into Roman lands, pillaging and raiding. The result was the utter devastation of much of northern Gaul, areas of which took nearly 150 years to recover from. The Huns were finally brought to battle again near Trier in 463. The Roman Emperor Julianus arrived with a large army at the city of Trier about three days before the Huns showed up. The battle itself has largely been lost to history except for the fact that it was a truly titanic massacre. It was said that the Mosel River was said to be clogged with gore and red with the blood of the slain for 2 weeks. The result of the battle was the death of Clothodus, the annihilation of a large portion of his army, and the destruction of nearly 2/3 of the Roman forces. However the Romans pull off the victory, no matter how Pyrrhic it was.
In the resultant chaos, the Vandal foderati and other Christian populations of Baetica rebelled. When the Romans sent what forces they could to put down the revolt the Vandals outmaneuvered them, sending a large portion of their fleet into North Africa, where they conquered the Christians of Carthage and the surrounding lands. This marked the permanent loss of North Africa and its rich food stores.
Over the next 120 or so years, the peoples of Europa have done their best to stabilize the borders of their respective kingdoms. Even with the Avar incursions and their invasion of Dacia, general peace has somewhat returned to Europe, in the West anyways. The Western (and by this point only) Roman Empire is a much changed beast from its former self. The Temple of Sol Invictus is the highest law of the land, having replaced many of the institutions that were held by the Emperors of the previous age. The current Emperors are usually the High Invokers (male) or Keepers of the Light (female). It is also the place where knowledge is kept, with manuscripts, scrolls, codesies, and other such items of learning kept there... usually behind locked doors/reserved for the nobility. The army has significantly changed its scope, from that of a professional infantry force to one based around cavalry, usually heavy knights and horse archers. So it has turned into a Holy Roman Empire of sorts. () The Thrungians have been slowly unifying over the past 100 or so years. The greatest of the tribal kingdoms are those of the Franks, the Ostrogoths, and the Dann.
Wall of Text of Doom is ... wall of text of doom. You might want to put at least spaces between your paragraphs... and probably a better idea would be condensing what you do have down some. Just my two cents.