I'm the king of Kievan Rus and have quite a bit of territory. My goal is to become emperor of Russia. As a step towards is to become a king of Chernigov Kingdom.
My inheritance law is elective gavelkind, the good thing is that MANY dukes (Grand Princes in Rus terminologies) and the current king of Chernigov kingdom are all from my dynasty: Rurikid.
The current king of Chernigov can get around 6k troops and has 22k troops of potential allies... Also, right now he is attacked by someone from Persia (vassal of Seljuk's empire) and losing a war.
I can attack now and start getting counties and duchies, but it will be a lengthy and bloody process to take all his territory (as he has quite a few duchies).
Another option I thought about is to elect the king of Chernigov to be a king of Kievan Rus. The problem with this is that nobody votes for "an encroaching foreign ruler".
I also thought about marrying his and my kids so his son would inherit my kingdom and I elect my grandson as a king.... but that's a too long shot.
Are there any better options?