At this point, it shouldn't be spoilers that Diabolos is the boss of the lost city of Amdapor. So here are the things to think about:

What is Diabolos? Obviously he isn't a primal, though he is a traditional FF summon. This begs the question of if there are primal like beings like Siren (Who may not have been a summon, but she was essentially tempering, albeit hers is more easily stopped or avoided). All we know is Diabolos was sealed in Amdapor and was somehow related to its downfall. What is his connection? What did he do? What caused his seal to weaken. Since Amdapor requires the main story to be complete, it's not unreasonable it has something to do with the Primal roar heard at the end of the main story.