So I've been playing this as part of my more general attempt to get into Paradox games (and after playing it and Victoria I can actually understand the first game I tried to play in the series, Rome). I played a few little games to get more familiar with the mechanics, the whole vassal system confused me spectacularly for quite a while for instance. Got my ass kicked several times before I realized how the levy system worked, etc. then I started up a Byzantine Empire game and have now reached the year 1300 and things have started to fall apart. Over the past several hundred years (started in the Old Gods time) the Byzantine Empire has been ascendant, retaking various areas and driving the Muslims back. Then finally in 1200 the Emperor retook Alexandria and was able to unite Christendom.
The Byzantine Empire is still probably the most powerful nation, but we are constantly stamping out peasant and heathen revolts, and have been for seventy-five years or so. The only time it stopped was to fight the Mongols. The Ilkanate was defeated soundly after 8 years and collapsed shortly thereafter into four seperate kingdoms. The Golden Horde conquered a number of duchies north of the Black Sea, then converted to Orthodox Christianity.
Unfortunately I didn't realize the game kept track of family lines, so genetics are not going well for the Imperial family at the moment. In a panic the current Emperor (whose wife had just died) looked for a bride from an area the Imperial family had never married into, and ended up married to the oldest daughter of the Golden Horde's Great Khan. So yeah, interesting alliance there.
The Holy Roman Empire, our most stalwart ally (read the one the heirs kept marrying into because I wasn't thinking) has suddenly, and without any obvious sign disintegrated in less than two years.
I switched over to that family to find out what was going on and learned that the Emperor and both his sons died within four days of each other (one natural, one accident, one assassinated) leaving a cousin with a rather weak claim to the Imperial throne in charge.
The Dukes of Poland decided to revolt for independence shortly thereafter. The new Empeor, likely loking to prove himself worthy led an army to put them down, and was promptly defeated, his army destroyed, and he was slain. With no male heir.
Full scale civil war followed. Now, five years later the Empire has completely vanished from Central Europe. Only one duke stayed loyal (Savoy) and even they have now been subsumed into the new Duchy of Prevence. The old Empress is now Queen of Bohemia (just having hit 16 last year) and Central Europe is a patchwork of states that hate each other.
Meanwhile Sweden is trying to overthrow the English Queen, the Emperor of Hispania is trying (and failing) to subjugate Portugal, the former Ilkhanate is steadily losing ground to two of the breakaway states, and Kiev has been completely annexed by its neighbors.
Oh, and Mecca has been completely converted to orthodox Christianity.