The vassals. Bit border-gore-y, but I am just manging to keep a lid on it.
Alania is a bit big, but they keep conquering land, so I keep letting them keep it. I'm also de jure drifting everything new into existing kingdoms. Should have done that more right from the start, but it's a bit of a pain. I STILL have not managed to get Wessex, I think, out of the kingdom of Asturias and back to England, because every so often, I find I have to go in and slap someone around for trying to nick it and break the de jure drift... I don't know HOW long I've been at that, and it's still only 50% of the way there. (England and Wales have managed to drift into the Roman Empire at this point...!)