The monarch dies, and by the rules of inheritance his crown goes to his eldest son… OR the monarch’s younger brother. A dispute about the sucession arises. The various nobles of the realm see an opportunity to be kingmakers, line up on one side or the other, and hilarity (not) ensues. Just how many friggin’ times has this happened anyway? The most cursory reading of European history seems loaded with examples.
Give us a couple for fun.
If I may interject, Alfred the Great faced a rebellion, albeit quite a minor one which was quickly put down, from the sons of his elder brother. Alfred’s father, of course, had said that all the brothers would take turns, Alfred being the youngest came last, after all his brothers were dead. His descendants ruled after him, Edward, Aethelstan and so on.
England: Richard III vs. his nephew, Edward V comes to mind.
As does the case of King John and his nephew Arthur, the son of his brother Geoffrey. Richard I (the Lionheart) had died without issue, and the next in line would have been his brother Geoffrey. However, Geoffrey was dead – but his son Arthur was still alive.