So ever since I first started Chains of Promathia way back when (nearly 6 years ago), the lore has always been mentioning emptiness but I never really quite got a clear idea of what exactly it WAS. The things that I was sure of was that it was memory (specifically forgotten memories?), and that it was Promathia's will. Anyway, today I went onto the Final Fantasy wiki (not FFXIclopedia) and did a little bit of reading. Basically, my understanding of it now is:
Promathia's goal is to reign over everyone through death, and emptiness is a manifestation of his will. Therefore, emptiness is the manifestation of complete death. The fact that complete death manifests itself as "memories" probably alludes to the idea that memories are the only things that truly remain of someone once they die. Thus, complete death would be when those memories are forgotten and thus turned into emptiness.
So... am I right?
(Here's where I got/extrapolated all this, by the way: http://finalfantasy.wikia.com/wiki/Promathia Obviously there are spoilers but this expansion is old as balls so I expect all of you to be familiar with the basics of the story anyway).