Started as a Polish Duke. First ruler died as King of Poland and Hungary.
I revolted and gained my independence while my liege stupidly declared a Holy War on one of the Pagan nations (bringing down ALL the Pagans on him), so he was already broken and fighting off Pagans. During my fight for independence, I captured the king in battle, giving me 100 warscore. After the war was over, I executed King Boleslaw - a capable ruler - giving the realm over to his young son. All of his vassals hated him. Meanwhile, I settled down for a while, built up my income with Castle Villages and Towns, successfully plotted to revoke one of my vassal's counties so I held all of the counties in my realm, then started planning the takeover of Poland.
A few of the king's vassals started a war to institute Elective Monarchy in Poland, and I provided financial support for their cause (couldn't join the war for whatever reason). That war took forever, but when it finally ended I had the most nominations for heir to the Kingdom of Poland. I successfully plotted to kill the king... but the Kingdom went to his brother instead of me (and they had autonomous vassals, so it couldn't have been crown laws disallowing titles to pass outside of realm).

Even though I was trading the top heir spot with the other brother, I decided I had to resort to more drastic measures. I saved up my money (had more income than the king, even though he had twice as many holdings as me... good construction FTW) so I could afford good mercenaries and started preparing for an invasion.
The Pope liked me, and after I gave him a little gold he freaking loved me, so I used the invasion CB for Poland, which was weak after its really long civil war about elective monarchy and another really long war of the Bohemian king trying to put Boleslaw's sister on the throne. I called in my Swedish allies to help, and the war quickly turned into mostly Swedish and mercenary troops doing all the work. After occupying all of Poland, I enforced demands, and every single holding in Poland became mine. I made new vassals in all the bishoprics and cities, then gave a couple counties and duchies to my heir to get under the limit.
Owning practically all of Poland gave me about 27 gold per month after the "Recently Conquered" modifier wore off. After waiting almost ten years for the recently conquered modifier to wear off and levies to refill, I asked the 80-something year old Pope for permission to invade Hungary. It was granted, and I went on to hire a buttload of mercs, call in Denmark, and curb-stomp Hungary with ease. After occupying all of Hungary's provinces, I enforced demands and installed new vassals in all the holdings, giving only one county to each vassal.
Then Zachary the Conqueror died shortly thereafter, a legend.
