"But still there comes a time when each Keeper knows death is near. At this time the Crystal begins to lose its ɹre and its strength. Then a new Keeper must be chosen, to take the old one’s place. The new Keeper must join with the Crystal before the old Keeper dies, so that its power is not lost."
The Choosing was an event held by the three clans of Maris to decide on a new Keeper of the Crystal, as decided by the Chooser.