I'd argue that making them too specified with that Covenant of Primus book was a mistake since it only ended up ditching accepted facts about the few defined Thirteen Primes up to that point anyway for the worst reasons. The most offending of them of course being how The Fallen went from basically the Transformers equivelent of Satan (always on fire, turned to evil from Unicron's temptations of power, etc.) to essentially a carbon copy of Prime Megatron who turns evil because of a love triangle. And then Optimus Prime being a reincarnated member is just completely stupid as well.

The Thirteen as a general concept works, but the full roster should never, EVER be filled out completely. Leave some members defined if only barely anything more than a name (Vector, Fallen, Maximo, Nexus) but leave it open for the various mythologies to fill in the blanks in their own way. These are mythological beings we're talking about, they should be defined just as much by untrue fantasy as hard facts.

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