In OTL, the period between 251-750 CE comprises the transition from antiquity to the Middle Ages in Europe, Northern Africa, the Middle East, Southern and Eastern Asia. In some places earlier, in some places later, great imperial civilizations collapse. Unable to contain and fight off invaders from their nomadic fringes, the Roman Empire collapses under the migratory pressure of Germanic gentes who flee from the Huns, the Sassanid Empire is conquered by Arabs, the Gupta Empire is destroyed by the Hephtalites, and the Xiongnu dominate over the Chinese heartland. The barbarian intruders begin building their own empires and kingdoms, but the differences are clear: Two rivalling monotheistic religions (Christianity and Islam) become state religions; they replace the cultural plurality and political-economic imperialism of Antiquity with pressure for uniformisation and religious crusades. Slave-owning economies develop into feudalist structures. The centres of population are no longer large towns, cities and metropoles, but villages and fortified small towns. Trade between Europe and Asia as well as Europe and Africa diminishes. A lot of book knowledge is lost and literacy is increasingly limited to the inner circles of religious communities.
In this timeline, the crisis of the Roman Empire does not evolve into the painfully slow decay into Byzantine insignificance, but leads to a revolution, which in turn leads to the construction of a second Roman Republic, where the functioning local structures assume a greater role, the power of the soldiers is institutionalised and channeled, latifundia are owned and run by collectives of former coloni and slaves, and the political institutonalisation of closer ties between the cities with their skilled workforce and their immediately surrounding countryside brings new impulses for technological innovation, increasing productivity and intensified cultural assimilation.
While in OTL, the empires are weakened by permanent inner power struggles and wars against each other, the Second Roman Republic keeps peace and invests in economic exchange with the Sassanid Empire. This allows the Sassanids to expand far into India. Large, prosperous and powerful, the Roman and Sassanid Empires can easily keep the Huns, Xionites, Kidarites, Awars, Hephtalites etc. at bay. In fact, the rage of the attacked empires turns into a genocidal lashing out, which depopulates much of the Central Asian steppe and postpones the threat from nomadic invaders until the arrival of the Göktürks.
In OTL, strengthened Germanic gentes, which the weakened Roman Empire can no longer contain on its long Northern borders, take over and establish new states - chiefly among them the Goths, the Franks and the (Anglo-)Saxons. In this timeline, Decius` war splits the Goths in two and absorbs one part, reducing the remaining Gothic Empire to a less dangerous enemy, until it is finally overrun by the Huns and inherited by a pseudo-Ostrogothic polity, which is closely associated with the Roman Republic. The Roman Empire can concentrate on holding the Danube line, until economic and military recovery as well as Hunnic devastation of the neighbouring Germanic tribes allows for a slow integration of the Eastern Germanic and Suebic population. The brunt of the Germanic onslaught hits the breakaway Celtic Empire, which, after several periods of chaos, copies the Roman political model and actively selects and integrates Germanic and Celtic populations and territories into their decentralised empire.
In OTL, the imperial model of monarchy and aristocracy appeals to the barbarian newcomers, who copy the concepts to a certain degree. In this timeline, it is the Roman model of rural collectivism, urban democracy and economic development which exerts great "soft power". The Celtic Empire experiences its revolution in the late 4th century, while the spirit of egalitarianism of the revolutionary days survives in the Christian sect of the Simonists, and its success later inspires the Mazdakists in the Sassanid Empire to hold out until they finally bring about the collapse of Sassanid rule in the 6th century. Simonism spreads among Rome`s Southern oasis-dwelling neighbours and from there further South to the limits of the tropical jungle, while Mazdakism reforms Eran and the disintegration of the Sassanid Empire sets the Silk Road cities of Bactria, Transoxania and Sogdia as well as the Indian satrapies free.
In OTL, transcontinental trade volumes decrease and the rise of the Caliphate cuts off Europe and South and East Asia from each other - the results are slow technological advances in both Europe and South and East Asia, and the emergence of new, albeit weaker trade routes linking e.g. East Africa to Arabia and Persia via the Indian Ocean - giving rise to the Swahili culture -, and Northern Europe with Byzantium and Asian markets via the rivers of Eastern Europe - giving rise to the Rus. In this timeline, increased global trade quickens technological advances and strengthens urban structures everywhere: in the oases of Central Asia and Arabia, in India, on the East African coast, in the Malay archipelago and in the parts of Europe which lie North of the Roman and East of the Celtic Republic and which are inhabited by Germanic, Slavic, Baltic and Ugro-Finnic peoples. The security provided by the great stable empires and the depopulation of the Central Asian steppe as well as the successful example of the Roman civitates causes wealthy city republics from Norway to India and from the Azores to the Tarim Basin to assert their independence . while in OTL, feudal lords battling each other concentrate more and more power over what is left to rule and petty kings turn into greater kings.
In OTL, economic structures become more static and local in this period. In this timeline, earlier developments of proto-capitalist market economies lead to great social inequalities, which further increase inner social dynamics. The centres of development - Rome and China - manage to "export" these problems due to their higher productivity. The inner destabilisation and the centre-periphery conflict potential which this brings about do not yet wholly unfold in this period, though.
In OTL, the religions do not permeate, syncretise and influence each other very much because religion becomes a matter of state coercion in the emerging Middle Ages. In this timeline, Zoroastrian beliefs like Mazdakism inspire new developments in Christianity, Roman and Celtic paganism syncretise with Germanic, Baltic and Slavic paganisms, Buddhism comes into intensive contact with several Asian shamanisms as well as with Arabian and North-.East African Christianity and with Bantu beliefs. Native religions of Germanic, Baltic, Slavic, Finnic, Uralic and Turkic backgrounds, which in OTL become marginalised and extinct, often begin to scripturalise towards the end of this period of the Abrittus timeline.
In OTL, the Franks, the Vikings, Varangians and Eastern Slavs are about to become new power centres in Europe. In this timeline, Roman and Celtic dominance last and the nations and tribes on their periphery either become absorbed, or develop relatively quickly at a similar pace and without many empire-building opportunities. Franks, Saxons, Norwegians, Danes, Swedes, Liuticians, Prussians, Curonians and Permians develop city republics and kingdoms, and so do the multicultural cities in Eastern Europe, which are linked to the developed markets by Ostrogoths, who have turned into Jews, seafarers and (inter-)continental merchants.
In OTL, Arabia remains an unorganised space, where rival clans compete for supremacy, until the Muslims unite the peninsula. In this timeline, the Christianized Kingdom of Sheba, which maintains a close alliance with the Roman Republic, unites the peninsula step by step.
In OTL, India is ruled by rival empires of old and new aristocratic dynasties, and the Indo-Aryan culture, together with Brahminism, culturally assimilates the Drawidian kingdoms of Tamilakam. In this timeline, Sassanids and trade contacts with Europe exert a much stronger influence. Tamilakam follows its own cultural path of development, distinct from Northern Indian culture, where Brahmins also play a less prominent role after repeated Sassanid purges and enslavement campaigns. Everywhere across the Indian subcontinent, city republics emerge in the 5th, 6th, 7th and 8th century, forming various alliances. Urban guilds ("shreni" in the North, "nikamam" in the South) assume a self-confident political role; castes do not become as rigid as in OTL, and economic power - not imperial power - is sought after by India`s elites.
In OTL as in this timeline, the rise of the Göktürk Empire in the middle of the 6th century is owed to the power vacuum in Central and Central-Eastern Asia (in OTL: after the Westward migration of Huns and Awars and the South-Eastward migration and disintegration of the Hephtalites; in this timeline: after the collapse of Sassanid rule over Transoxania and the general depopulation of the Western and Central Eurasian steppe in the 4th and 5th centuries). In this timeline, the power void is even larger, and Göktürk rule spreads even faster, amassing more wealth because of higher Silk Road trade volumes. But their quick and glorious ascent also prepares their even quicker and more dramatic downfall. Like in OTL, Chinese policies of divide-et-impera break up the Göktürk federation, but the emerging smaller federations are still powerful, and - prevented from migrating into the pontic, cis-Caucasian and European space, they begin protracted wars against each other, at the end of which they are degraded to mercenaries under foreign rule protecting the merchant caravans along the Silk Road.
In Central and Southern Africa, Polar Eurasia and Japan, the differences between the timelines are very limited. The differences concerning China cannot be explained along great and logical lines, they must rather be attributed to "butterfly effects", the most import of which may be considered a second Point of Departure by the more skeptically minded: Yang Guang, crown prince under his father Emperor Wen of Sui, is killed in a battle against the Khitans. His older brother Yang Yong becomes Emperor Yang of Sui and proceeds more cautiously against Goguryeo, finally defeating this important enemy in the East. Without the chaos after Sui`s defeat against Goguryeo, the ambitious reformist Sui dynasty does not fall apart. Instead, they begin to slowly transform the Chinese Empire into an absolutist monarchy with an omnipotent government which meddles in almost all spheres of life - the starkest contrast and alternative model to Roman democracy. China and Rome do not yet see each other as rivals or enemies in this period of the timeline, but it seems most logical that these two powers will shape the world in the next centuries (9th, 10th etc.), too.
The history of Australia, Oceania and the Americas is unchanged before 750 CE.