Ages of Doubt

The Age of Doubt was when the Covenant had a crisis finding the Halos. The Covenant then began to doubt the word of the Prophets, due to the fruitlessness in their search for Halo. The Grunt Rebellion probably occurred during this period. This event resulted in the creation of an Arbiter to suppress it, resulting in his death. This Also could have been the time when the Taming Of The Hunters occurred, also resulting in the need of a new Arbiter.

Note: There could be another Age of Reconciliation following the Age of Doubt, as the Grunt Rebellion ended and they were reconciled back into the Covenant.