The Ageless Bond
There is a Creator. It (for the Creator cannot be confirmed as adhering to a single gender) created this and every other world, and everything on them. It did not create them all at once, but rather by slow and careful guidance. When the creation came to culmination with the Great Dawn (the first self-aware human beings), It selected an individual to become a leader of men, and another individual to become a protector of men.
The leader was a god, and the protector was a guardian. When the god died, his soul was transferred to the body of a newborn baby. When the guardian died, his soul was destroyed. The Creator then made the decision that no guardian could die unless killed, and instead of choosing a single guardian, It chose a handful. For a time, things worked well.
Until men started becoming gods of their own volition. Aristocrats and peasants alike were gaining the ability to govern the laws of reality, doing so with reckless abandon. The guardians, with the knowledge that it was their job to protect humanity, opposed the gods, and a small war ensued, in which most gods were killed. After the war, it was decided that the guardians needed a place where they could live and exist without the prying eyes of humanity upon them, a home where they could put limits on the gods that would soon walk the planet again.
And so, in the middle of a desert land, the guardians set to building the Silver City.
The guardians, however, realized that the world now required the gods to keep things running smoothly. Without gods, reality was fragile and unstable. When the gods began to remember themselves, the guardians suggested an agreement: No guardian would ever kill a god without provocation. In return, no god would ever abuse his or her abilities.
This agreement was called the "Ageless Bond."