With the age of the Covenant empire, how many other civilizations do you think they had contact with? There must have been species that did not join as a fringe group but instead maintained some sort of trade relationship or treaty with the Covenant.
Not sure… but it’d be cool if there were to be a spinoff game or novel on the covenant before the human-covenant war about fighting other alien species that we don’t know of 
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Well I mean civilizations that would have proven equals to the Covenant. There must be other tier 2 ones out there, unless the Covenant truly has been the strongest one since the forerunners.