There have been hundreds of challenges in the past, each only successful in removing the current tyrant.Each time the great dragon dies, he is replaced by his sibling, or one of the hatched. The hatched are the ones who come from the eggs that hatch whenever the great dragon dies. Each dragon has a sibling, and if they're sibling is dead, then one of the two dragons that hatched at his death will take over. The main flaw in the Challenge is that none think to attack the eggs.
The Challenge occurs every few hundred years. A group gathers at the top of the pillar, and reads this stone. They then make the signal, and go off to kill the current dragon.
Once, there were hundreds of Dragons, but a disease spread through the land and killed all but two and their eggs. These two took over the land, and made it so that they're descendants would rule. They did not count on the Challenge, however, and were overthrown. Now,it is a tradition for a group to challenge the ruler every few hundred years. They won't be attacked until they have made the mark.
To make the mark, the challengers must all make a cut in their arms, and let the blood flow into the basin beside this stone. Then one must set the basin aflame, and put in some of the ice next to the basin. This will turn the flames blue, signifying that the Challenge has been made. After the Challenger's make the signal, they must travel to the bottom of the pillar to find a boat waiting for them. They must take the boat into the fog, and go to the Dragon's Island.