Not so much a death as a rage quit.
I was playing as Poland. It was my first game, so I'd made some mistakes and paid for them dearly. Fortunately enough, I was beginning to get a handle on things. My ruler, the most recent in a long line of underage children, has just became an actual man at 16, complete with beard. Suddenly, Bohemia's ruler decides that he'd like to annex me, much like his father had tried and my grandmother over in Kiev. I don't understand demesnes for squat, so the country was split between me (Greater Poland and parts of Pomerania) and my vassals and brother. Bohemia mounts a 1600~1900 man force and starts consolidating it, and my levies only yield around 800 men. Tribal groups can't call out their vassal's levies, so I saved and tried upgrading my tribal organization. That allowed me to call up warriors from across the realm, a good 3000 or so men. I bravely bash my larger numbers against Bohemia's smaller numbers like a large cat falling onto a smaller cat. Joy overtakes me. Look at me grandma, I'm protecting Poland!
I'm doing it! It's happening! Not five seconds watching these numbers play out before I find that my fresh and shiny new capable ruler has received a blow to the head and is incapable. It's not the brain damage to my ruler that made me rage. It wasn't the revolt that sprung up in Plock or so that made me rage. It was the message, barely five minutes into adulthood, that stated that the same man who was my regent five minutes ago was my regent again, but it wasn't going to be only for six years this time...