I noticed after you beat lost city you can talk to E-sumi-Yan and Raya o Senna for a lil bit of extra dialogue.

The following is me just speculating out loud.

E-sumi talks about how Amdapori's enemies summoned diabolos with dark magicks, but Diabolos was sealed by Amdapor's mages in the end. So assuming he isnt a primal I am curious if he can come back since he had a body and itsn't made of ather like primals are.

E-sumi also called Diablos a king among demons which makes me wonder if there is some type of civilization of hierarchy in the void.