The 'Four Fiends' that we see in the 6.1 cutscene seem to have been around for 10000 years or so. I suspect that it's a case of being 'cursed with immortality', and there might not be a way for lost souls on the Thirteenth to actually return to the lifestream at present.

I do want to know more about aetheric corruption and how it relates to the rejoinings and calamities to date. For example, what prevented the First from instantly becoming like the Thirteenth when Ardbert killed Mitron and Loghrif? Does the element in question actually matter, or could you create a 'Void of Light' filled with sin eaters?