Damn... definitely never seen that in all my 1000hrs. I'm going to go out on a limb and assume what must have happened is that the Abbasids failed (or was at least down for the count for a period of time) and then by some miracle the Byzantine AI decided to holy war the Levant instead of its age-old obsession with Persia and the Steppes. Then I would think somewhere in there a Byzantine vassal in Italy was able to take Rome because the Papacy was out of the picture due to the Fraticelli heretics.
Closest I've ever seen was a game I had ages ago where a random Lombard-turned Italian dynasty usurped Lombardy and then inherited Byzantium. They managed to grab Rome and I think Antioch, but never went deeper into the Levant to get Jerusalem or into Egypt to get Alexandria.
I watched the entire thing unfold. It's quite similar to what you said.
Muslim Side:
Abbasids lost ALL of Persia before 800....
They lost Mesopotamia soon after.
They put all of their resources into North Africa and the Steppes.
Soon, they lost the Empire to the Muhallabids, who somehow lost the Caliphate.
The Muhallabids lost the entire Arabian Peninsula, but captured Nubia. The Muhallabids stretched themselves too thin with the annexation of Nubia, and so they lost the steppes. Their steppe empire was HUGE, like half of De Jure Russia and De Jure Tataria.
But, then the Muhallabids lost the Empire to a Decadence revolt, which made a bunch of people in Palestine and Levant to leave the Empire.
The new Emperors (Forgot their name) had a ridiculous amount of Decadence, and so they had very little troops.
You know the rest.
Byzantine Side:
The Byzantines had a good start, the Slavs in Croatia were annexed before 900. The Serbians, however were a problem. They had made a kingdom that stretched from Serbia to Bulgaria! They converted to Catholicism, and were a problem. The Byzantines SOMEHOW annexed them. I'm not sure how?
Anyway, with the Balkans secured, the byzantines smashed Lombardy and captured all of De Jure Sicily.
They then attacked Armenia, and won easily.
I think they must have fabricated a claim on Rome, because they annexed it in the mid 900s.
They kept expanding further into Armenia, and secured Antioch by 1000.
About then, the Muhallabids took charge, the Byzantines turned North. They annexed Crimea.
They lost Rome, and didn't gain much land until the collapse of the Muhallabids.
They went berserk and destroyed the entire Levant and Jerusalem before 1050.
After that, the Byzantines and the Abyssinians attacked the Empire from the North and South. Abyssinia secured Nubia, the Byzantines, Egypt.
The Empire's few Syrian provinces were ripped apart by their fellow Muslims.
The Byzantines captured Pannonia, (AKA the southern half of Pannonia), and then they attacked Rome again. (They used Holy War against the Fraticellis.)
I still can't believe it...
