The Law of Godly Requirements

The Law of Godly Requirement was not an original law of the universe, it is the only law that has ever been added to the consternates of the universe. Ahura Mazda, the wise Creator, decided that Gods should not rule simply because they were created to fill the place, as time passed their children may stray from the perfections that they had birthed. This belief of Mazda was evidenced by The Falling and the subsequent events. 
The Law states that a God, is an immortal and cannot be killed, yet if a being or different entity is more worthy of a position then they have the power to defeat.