It's now 960. Karlmann, son of Arnulf, is king of a unified Bavaria and East Frankia. He's getting on in years, but he's not finished expanding his territory yet.
Because he's head of the Karling dynasty and very, very pious, one of the things he can do is ask the pope to give him a claim on another Karling's titles. So that's what Karlmann does. The pope gives him a claim on the kingdom of Lotharingia.
Lotharingia is tiny, compared the the united lands of Bavaria and East Frankia. But it's strategically very important to Karlmann's end goal. One short war later, and he has successfully pressed his claim, giving him control of enough counties to declare himself Emperor of the Germans and to form Germania. All he needs is a little money to give the affair the pomp and grandeur it deserves. Once again, he turns to the pope for help, and once again, the pope is happy to provide: a loan, this time.
Six years later, Karlmann dies, aged 80. His son Matthias, already in his 50s, takes over. He's the one with the eyepatch. Matthias has six daughters, but his wife has just given birth to a son, Kuno. He knows that at least some of his kids were probably fathered by his brother Arnulf, who has been getting around--with both the ladies and the gents in court.
When a king takes the throne in Crusader Kings, there's usually a bit of instability. Every vassal has a penalty to how much they like the new king, based purely on the fact that the king is new. You need to prove yourself. Matthias is well enough liked because he's got a career as spymaster under his belt, and his father was well-liked. No major factions arise against him. His wife gives birth to twins, which he suspects aren't his, but when he asks her nicely, she says "No, dear, we definitely conceived them together, that one time, remember?"
Meanwhile, elsewhere in Europe: Sweden.
So Matthias enjoys a mostly quiet reign and even adds a few small territories to the empire through vassalization rather than conquest. But then, after reigning for just six years, Matthias dies too, and now there's a problem.
Kuno takes over. He's 9 years old. His father didn't reign for long. He has powerful vassals who are his uncles and cousins, all with claims on the throne. As rulers, children have fewer options open to them because they are, well, children. They are highly dependent on the stats (and motivations) of their regent, which the player does not control.
Kuno tries to make his life easier by giving a kingdom title to Uncle Arnulf, who might actually be his real dad, but that backfires. Once Arnulf gets a title, he wants more.
Soon, enough vassals have banded together to demand that he hand over the imperial crown. He refuses. A civil war breaks out. Young Kuno is losing ...