Ages of Conversion
From Halopedia, the Halo wiki
The Age of Conversion is theorized to be the time that the Sangheili's and Prophet's began incorporating the other races into the Covenant, either through conquest or religious conversion.
Background
During the Age of Conversion, the Hunters resisted conquest more strongly than the other races which resulted in the creation of an Arbiter to help “tame” them, though the Arbiter was eventually killed. The Hunters defeated the Elites on the ground but were forced to surrender under threat of extinction through glassing. It is also possible that the Grunts were one of the species that was conquered rather than converted. Almost nothing else is known about how the other races became part of the Covenant.
As the Covenant would have only picked 'worthy' species, it is likely that these beings would have, at some time in their history, either met the Forerunner's, or evolved on a Forerunner planet. It is possible that the Covenant had found us during this time, but it took them until the Ninth Age of Reclamation to decide whether or not we were worthy.
