I read this novel years ago.
The story was about a kingdom where the future king is chosen by the king having a certain number of children, then as the kids grow up, they have to compete against each other in ever more deadly trials. At one point the kids have to swim to the bottom of a lake and recover a flower, another trial is climbing a mountain after a feather I think.
The next to the last trial is a quest they have to undertake and the main character has to recover some sacred stone of Boah or Baoh. Then he elects to go save some princess after he returns with the stone even though it means helping his brother and he has to cross some deadly swamp to do it. The final trial is a trial by combat with all the living heirs and last man standing will inherit the kingdom.