How in the world did you get so big so fast? I started as Duchy of Provedence and am only 1/5 the size you are, and I'm in the year 1000 AD! Give me tips oh conquerering master!
Its not really an accomplishment as I started as Charlemagne. So start as Karl of West Francia and 2 years in the game Karloman of Middle Francia will die and you can easily dominate everything after that.
True, Karl, aka Charlemagne, gets Middle Francia basically for free if his bro Karloman properly dies from the events.
Plus you get an event to conquer the Kingdom of Saxony. edit - but not Bohemia or Bavaria, thanks Aardvark.
edit - Oops, there is no event for Bohemia or Bavaria. However, Carloman's wife flees as a result of the event chain between Carl and Carloman. Charlemagne gets a claim on whichever semi-random location Carloman's wife flees to. That could be what I was seeing.
Plus if you marry the right Lombard Princess, you get an event to divorce her. From that it seems you get a claim on Lombardy. The events seem to overlap in strange ways though, so I'm not really clear on the event chain. Do you NOT get the claim on Lombardy if Carloman's wife runs elsewhere? Regardless how it happens, you have to own Lombardy to form the Holy Roman Empire.
(...and in 2.5.2/Conclave, you get event-based expansion for zero Threat penalty, which is a new game mechanic vaguely similar to Aggressive Expansion from EU4)
edited because I was wrong about which claims come from events on specific countries.