The British Isles have been a strange story. Ireland unified itself before 1000 and has been more or less together since. Scotland has progressively lost land to basically everybody, and at one point there were nomads in Glasgow for some reason. The Anglo-Saxons got the short end of the stick; they were progressively eaten by the Franks until England basically became a second front to manage when the Franks went to war, sort of like colonies in EUIV.
Francia... hoo boy, Francia. The troubles started when a faction installed a Merovingian in place of Karloman, denying Charlemagne the throne and sowing the seeds for Europe to become bordergore hell. Burgundy popped up somewhere down the line, which wasn't too bad; it cut Austrasia in two and had an exclave in West Francia, but I've seen worse. But then some damned fool created Frisia and turned Western Europe from French-German slurry to multicultural vomit. To add insult to injury, the Dutch culture and language has been completely eliminated from the face of the Earth, so Frisia is something of a moot point now.
The pagans have had their usual nonsense where Scandinavia gradually turns Slavic for no reason and some random chief invades half of Northern Europe. Sweden and Denmark formed, then Denmark conquered Scandinavia up to the Kola Peninsula, but never touched Finland. Sweden and Norway became independent from it anyway, and not much interesting stuff happened for quite a long time. There were a couple of Slavic pagans taking the thrones from time to time, but within a king or two they were Catholic, which was probably egged on by Burgundy taking Østlandet and some land near it. Norway has now taken most of the Swedish and Danish land they can get their hands on. Meanwhile, Saxony fought off Charlemagne not once but twice, then was conquered by Pomeranians and has remained in Slav hands ever since, converting to Catholic about 150 years ago. Finland formed thanks to Estonian efforts and Lithuania's been a thing for quite a long time, but that's about it.
The Khazars took a quick lead in terms of Tartarian and Russian land. They burned down 60% of Russia and almost wiped out Russians, but thankfully they kept a few hanging around who were able to rebuild to a small degree afterward. Pannonia was conquered by Poles, then split away from Poland and became Catholic, then was conquered by a khagan by the name of Zagreb; Zagreb immediately settled and only owns small bits of territory outside of Pannonia, which is still ruled by the King of Pannonia (although the King of Pannonia is now a reformed Tengri-ist.) Then the Mongols showed up and promptly swallowed Tartaria whole.
Byzantium and Lombardy are weirdly co-existing. The Byzantines seem to want to go for a Roman restoration, but don't appear to grasp the fact that conquering Lombardy via claimant (and taking Venice) would allow them to do so immediately. They are instead content to conquer Wallachia and Mesopotamia for no particular reason or gain. Lombardy is the guy who wants to be friends but never gets invited; they've taken a couple of bits of Africa from time to time and have a few little counties and duchies in Bordergoretopia, but that's about it.
Charlemagne tried to halt the progress of the Umayyads by extending protection to Asturias, but quickly lost most of the territory there. That territory was then reappropriated by a Umayyad splinter faction, who actually did halt the Umayyads for about a hundred years before they were conquered anyway. The Umayyads went yellow and were almost immediately overthrown by the Qashids.
Ramadan was a creatively-named dynasty that overthrew the Abbablob within two decades of the game starting. This allowed Egypt to leave the influence of the Caliphate; they never went back and were the primary opposing force against Ale, a black Viking who conquered Nubia. The Ramadan were primarily opposed by the Shi'a Caliphate, which won an early-game war for Syria. The Ramadan were nearly defeated by the Shi'a, who built a long, stringy empire from Aleppo to Khiva, but were beaten back by the Ramadan (who I believe lost Jerusalem and Antioch due to this.)
Not pictured is the Mongols sacking and looting India; ages ago, an Indian guy conquered a strip of land up past Turkestan, but the Mongols conquered the entire thing and are currently using this corridor to burn down Rajastan.