Question: This global-spanning empire is divided among five royal families, each headed by a prince. The emperor himself leads the empire, with the five princes serving under him. While they owe fealty to the king who has absolute authority over the empire, these families operate semi-independently and control their realms directly, as well as their own security forces and economy. The king holds all secular power by law. However, his right to rule is given to him by the gods, with priests of the theocracy speaking on their behalf.

The selection process of becoming king is administered by the priests themselves in order to prove the future king's worthiness. A series of trials are conducted in which the heads of the five families compete against each other. These contests vary with each selection process, and these princes can participate themselves or elect a champion from their realm to compete on their behalf. The winning prince of these contests is then elected to the position of emperor, and rules until he dies. The four losing princes are ritually sacrificed to the gods. This system is meant to prevent any disputes between rival families as to who is the rightful ruler, and to keep the empire stable.

However, it is possible that a prince or several princes may simply not accept the results. They may lead their families into open rebellion to avoid their honorable fate of being sacrificed for the good of the empire. This is unacceptable, for it would tear the empire apart with various wars.

How would I prevent this from happening?

Answer: 
If I remember correctly, in ancient Japan all the daiymios had to send their sons to reside in the Shogun's castle, where they were held as hostages, as a warranty for the underling's fidelity. The practice was called [Sankin-kotai](https://en.wikipedia.org/wiki/Sankin-kotai), thanks to Jay Carr for pointing to the reference.

The emperor can use a similar system. Each family has to give their heirs into the complete control of the emperor. Rebelling to the emperor and his designation results into the beheading of the family itself, as all of the heirs will face death.

Be faithful and you lose just one member. Betray, and you lose all your descendants.
**Change Who can Participate in the Trials**

The death trials still doesn't seem like a fair deal, like you’re the head of a family you train for years, just for that one rich family to hire the best mercenary in the world to curb stomp you. If I was one of prince’s I would rebel just so I would not get killed and install an elective Imperium like the Holy Roman Empire (like it’s a one in five chance to win and if you lose you die). It would also make your empire unstable as well; you kill off every head of state EVERY TIME your emperor dies (also I would be pissed if my dad got killed and the new emperor was responsible). 

What would be better if you still want a holy death trial is open it up to any volunteer (could be just nobles and or commoners) so anyone who wants to risk it for the biscuit can, instated of forcing a select few to do so. Like well-known champions get sponsored by the families and if they win then they will carry favour with the next emperor. Could open up a lot of possibility’s for story telling as well, like a minor noble becomes a candidate and everyone did not see it coming or a rare sight an actual prince does the trials and every is like wow.

if i was a prince i would be cool with that and if my son or i "volunteered" then it would seem far more holy then force us to do so. i would not be so pissed if he or i lost because both would have known what they were getting themselves into