The Truce Edit
The Truce applies to humans and humanoids who live in cities.
The Truce is a worldwide agreement that keeps anyone from seizing ultimate power. It's similar to our Geneva Convention. No one is permitted to rule more than one city at a time. It doesn't prevent nepotism, and plenty of cities are owned by the sons or daughters of nearby rulers.
It also does not prevent takeovers. If a ruler wants a new city, he can abandon or surrender his old city and try to take the new one. If a commoner wants to be a ruler, he can raise an army and overthrow a city. Colloquially this is called the "Survival-of-the-Fittest Principle." Commoner-born rulers are commonly called "fit rulers."
It's not known when the Truce was made. It seems always to have existed. Some historians lump it in with the arrival or creation of the Beasts.
(Work in progress)