It's only 52 years into the game, but Cumania
(Havuksingila dynasty, unfortunately converted to Cuman, I like countries not having their default culture) already controls what feels like a third of the map
(sure, the steppe has big provinces, but...), has reformed Tengri and I'm sitting right at their border, maimed, over 50, still without a nickname (even if it's for my arbitrary trait, give me a nickname already!) after 15 years as independant Grand Prince of Kiev and 12 more as first and so far greatest king of Ruthenia
(and some years as a count subjugating independant counts, my character's father died the single county count he started as), awaiting a succession crisis, slowly chewing away my Lithuanian and Bulgarian neighbors while Cumania spams Invasions and Hungary still has 30k troops. I'm scared. I can probably reform Slavic within ~15 years
(die, subjugate Rus, conquer Birlad in the meantime, would do it now if I wasn't an ally in a war with Bulgaria as an ally on the other side) but the horde will get me... Honestly, I wouldn't be surprised if they invade Persia next and form the Persian empire before Slavic is reformed. Or Mordvins->Tartaria, of course, but I expect better than that.