Fallen Assassin

When a prince is 7, the king and queen hold a "matching." The prince can be "matched" with any girl between the ages of 5 and 9 in the country. This girl then gets taken to the castle and is treated in the manner a princess should be treated. Within the year, she is sent off with an official to be taught how to run the country with the future king.
When she is 16, she is reintroduced to the castle and future husband. By the time she turns 18, she is to be married. This is the custom of all the countries, an agreement made to have new blood in the line.
A secret society of pure blood royals called assassins are trained to kill royals who break the rules or royals who they do not see fit to rule. They will assist the countries rulers to make sure the country has order.
These assassins are also the ambassadors of the country and make peace with other countries to prevent war. They are cruel and will show no mercy, all the royals know not to enrage them, or all hell will break loose.