A kingdom where, if there are multiple princes (princesses can't rule unless they have no siblings), the princes fight to the death in the arena to decide who gets to rule. A simple way to make sure your brother cannot make a bid for the thrown, and keeps the royal blood in one line.
In the middle of a desert in an oasis is a single prospering oak tree. Near the tree is a large pool of water. Drinking from the water is said to give you eternal life but really only extends your life for as long as you continue drinking from it. Those that have found it and left, have never found it again. Some search for it still.