See, this is why I approve of a "revoke immortality" demand when I use the current incarnation of a certain mod. I don't like it when my mortal "thralls" become immortal "rivals". What else should I call my vassals after the current ones have lived a few decades without aging a day? Anyways, attempting to kill immortals is one thing, waging a war across two and a half centuries is problematic...
He fought the Karlings for ~240 years from England and still is. Let's just say that multiple immortal kings and emperors is a challenge on its own. Probably shouldn't further complicate things with immortal vassal children, but I can actually manage them rather easily with all those mentor and dynastic bonuses. Well, that and they like to prey on each other. It isn't in their interest to cooperate but when dad steps in and tells them he'd kill everyone if they can't play nice at the wedding to wife number eight well...
For the record, I only had one of his wives killed (adultery). A few died in plagues and the rest of age or stress. He's on number eleven now and it'll be a shame when he goes. Britannia won't be united for long after that eventually happens. As much as I'm looking forward to the next DLC, I'll still miss the old man if he survives until launch.
I played a non-immortal (and Catholic) character who lived into his 70s, and upon his death, he had had (I think) 13 wives. 14 if you count the fact that he had children, but at start, didn't have a wife.