One of the issues for Kings was the huge problem of what happened after the King died. Too often the transition to a new King was endless warfare. One solution was the eldest son. And the authority for this was in the doctrine of the divine right of kings, which often worked so long as the eldest son was generally deemed legitimate, and in the high ends of aristocracies there was whole breeding program, which over the years got more and more complex, occasionally producing a few somewhat unstable personalities, or congenital idiots if you prefer. For example the Austro-Hungarians did reckon the oldest male offspring of a king or prince who erred by marrying a commoner wasn't really entitled to come under the category of the Divine Right of kings, royalty-wise his eldest son had no chance of being anything remotely divinely ordained. A Royal Hapsburg who chose a non-registered woman to marry wasn't really allowed to be seen in public with her, unless the event was military related. Arch Duke Franz Ferdinand, of First World War fame, had married an unregistered woman, she was also killed in the Sarajevo assassination. That visit to the provinces was considered a military matter, which was why the Arch Duke's wife had the misfortune to be there.
The Acholi people in Eastern Africa had the same sort of problems when their Paramount Chief started getting doddery. Who would take his place when he died, and how best to make the transition to a new Paramount Chief with as little public trauma as possible. Their solution was in my view rather brilliant, it covered all bases and worked pretty well. The close advisors to the Chief would generally inform the populace that the old man was on his way out, which meant all the kings male children would begin to try and kill each other. They didn't get into the equivalent of a Roman Arena and make a spectator sport of it. Instead the male children would go off into the wilder places, gather their supporters, all of whom would be promised easy work in return for loyalty and through a combination of bloody violence, traditional megalomaniacal political ambition and clever political maneuvering the Paramount Chief in waiting would emerge, so that when their father died, there'd be no debate around the transition of power. Most Acholi never had to get involved, they went about their day as though nothing was happening.