Ages of Doubt (Earth-2042)

The Ages of Doubt were eras in Covenant history where the effects of internal or external conflicts are dealt with. The last Age of Doubt, the 23rd, succeeded the 39th Age of Conflict, when the Unggoy Rebellion destabilized the Covenant, and lasted until the alliance with the Confederacy of Independent Systems and the deposition and replacement of the Hierarchs. This age was succeeded by the Ninth Age of Reclamation in 24 BBY.

Source

 * https://www.halopedia.org/Ages_of_Doubt