Starting as Duke Mutimir of Raska in 867, I quickly expanded my borders to be able to form Serbia. I also converted to Krstjani. Initially this play-through, my aim was to unite the south slavs and form a strong Serbian empire which also controlled Hungary. I thought that playing Krstjani would be fun too, as the regional Christian faith. Sadly, early on someone used Accuse the Krstjani of Heresy, and my position became untenable. Thus, whilst Mutimir died a Krstjani Serbian king, his son, Prvoslav I was forced to convert to Orthodoxy.
Prvoslav I and his son Porin didn't accomplish much of note. With the strong Croatian dukes to the north; a border with the Eastern Roman Emperors to the south; and powerful Hungarian and Bulgarian kingdoms to the east; not the mention that just across the Adriatic Italy had united under it the Burgundian, Lotheringian, Dutch, Aquitaine, and East Frankish kingdoms under it; Serbia was hemmed in and looked likely to be trapped as a regional power forever.
This all changed when Prvoslav II took the throne on the 12th of July, 946 at only 15 years of age. Early on in his reign, the Bulgarian and Italian kingdoms were dissolved due to internal struggles seizing the opportunity, Prvoslav waged countless wars against the now-independent Bulgar dukes to bring them into the fold. Using these gains in the East, he turned his army north and one by one captured the Croatian dukes too, until finally the last holdout in Dalmatia willingly accepted vassalage of the now nascent Croatian Empire. Lastly, Prvoslav II declared a holy war to capture the Kingdom of Hungary from the Magyr heathens. Prvoslav II died on the 11th of September, 1007. He inherited the crown of Serbia at the age of 15, but by his passing, he had added the crowns of Bulgaria, Croatia, and Hungary to his list of titles. At this point, the Emperors of the East were exceedingly jealous. Not only was the kingdom to their north growing in power, but they also controlled vast swaths of territory that were, in their view, rightfully theirs.
This growing threat resulted in a general policy of the next few kings of marrying a Byzantine Princess. The empire after all was stable and showed no signs of imploding as the Abbasid caliphate had just done. Of all the nations in Christendom, it was the most stable, having never experienced a civil war in the past 150 years, This meant that a marriage to a Byzantine Princess secured 50+ years of peace, so was incredibly valuable.
The next interesting King was Mircea, for it was he who finally wrapped up the last of the South Slavic kingdoms. On the 13th of November, 1089, he then managed the impossible and crowned himself emperor. Aged 67, he died in 1095.
Following his death, due to the unfortunate passing of his son, newly established imperial law which disallowed female inheritance, and a tradition of sending spares to the monastery, a succession crisis ensued. The Empire bounced around a series of wear rulers who had no surviving male children and did all they could to simply keep the empire together. It was not until the 10th of May, 1127 that a branch of the family would emerge which would survive. Emperor Vuk, whose last male ancestor of the main line was King Porin, his great-great-great-great-great-grandfather. Born the son of the Count of Hotin, found that he was now emperor of all Serbia.
His son, Konstantin 'the Great' married the daughter and only child of the Byzantine emperor, securing his children's right to both thrones and paving the way for his son, the grandson of Emperor Vuk, Teodosiji I, to conquer all of Italy, dismantling the papacy and restoring Orthodoxy as the one true faith. After which, he restored the Roman empire, forever fusing the Serbian, Byzantium, and newly conquered Italian imperial titles.
The Empire is currently ruled by Vulcan, grandson of said Teodosiji I, Car of the Roman Empire; Despot of Serbia, of Hungary, of Italy, of Thessalonica, and of Epirus; Duke of Raska, of Dulkja, and of Thrase; count of... He has completed all tracks of Hastiluder and is so wealthy that he can't give his money away fast enough.
These are the current borders of the empire. In addition to the kings of Anitolikon, Krete, Syria, Sicily, Jazira, Georgia and Egypt which all exist within the empire, the kings of Bavaria, Blemmyia and Lotheringa are all dynasts. We almost had Burgundy, but my Brother, who was betrothed to the Burgundian daughter and heir, became a mercenary leader and as such never married her or gave her children before she died.
As I mentioned, Orthodoxy is doing pretty well in this game. So well in fact, that there are two Orthodox Mongol kingdoms. I've not got a great way to show it, but the Sunni caliph is also a vassal of a vassal
In case people are wondering, the only game-changing mods I have active are VIET, Petty Inheritable Traits, and CFP. I'm not quite fone with this game yet, I plan on conquering Iberia, Tunisia, and taking a chunk out of the Jahangirids before probably converting to EU4. I also have a son married to a Gujari princess (Kingdom of Punjab), so I might see if I can place one of his sons on that throne to establish my dynasty in an Indian Kingdom too.