I had an heir who was Stressed, Depressed, Possessed, and Lunatic and my Council insisted on him as the heir. I also was gonna lose one Kingdom I had taken over (still getting back into the whole "how to cope with Gavelkind" thing!). So I sent him off in the lead of a stack of about 300 mostly light infantry (this was like 850 AD) to be euthanized in a meat grinder of Avar or some such tribal nightmare horde from the east (playing Bohemia) who was antagonizing one of my allies down in the Balkans.
His regiment got whittled down to about 70 and retreated all the way back to Praha. I sent him back for another round. This time his regiment was wiped out. So I raised the available levy from the same county (I think it was Plzen) and sent him back to the front . . . *musical interlude*
He dutifully led his small platoon back into the fray . . . and once again, wiped out. But Stromlinkslav would NOT die!
I raised ANOTHER levy and sent him back again. This time only around 35. Same thing. When he got back to Bohemia I had a look at his character pane. On top of his preceding malus traits, he was now Severely Wounded, Scarred, suffering an infection, and one or two other ones I can't recall. Confident he would drop dead soon. I played it cool from here, satisfied that my efforts at fratricide by military service would prove successful. But no. Three more years passed. Within a matter of a year he was all healed (well no more wounds or infection, though I think he did have an additional "behavioral health disorder" like maybe drunkard.
I think it was at that point I gave up and not long after my old heroic avatar dropped dead at 64 without warning and *BAM* Mr. Possessed and Obsessed Cannot Die was in charge.
He was wearing a cow skull on his face (damn I should've got a screen cap of this . . .) and as it turns out he was Demonic or something!