Read more fan speculation on the story from fire emblem forum:
Something I realized that might time this as a prequel but between Sothis fixing the world and Nemesis' genocide . This was a quote about more detail about what happened in Zanado and the plan with the TWSITD
- The first part fits with what we know about the setting as it seems like the Nabatean is leader of the city (Divine Soverign).
- The second part is where I think Dietrich fits in, he says I must cull (it) with his blade might be after the Divine Soverign and part of TWSITD plan to overthrow the Nabatean (again). And that he is fighting someone obviously with advance tech makes he think he realized that he was in the wrong and "defect" from TWSITD control. Although this is with the assumption that Nemesis wasn't the first victim and they tried that is other more distant places first before going with the Nemesis plan which might explain why Sothis is alive.
- It can kinda also tie in with Lena if it turns out that she was from one of the settlement that wreaked havoc on by Dietrich because of TWSITD's plan so she is after him for revenge.
-Cai is the biggest ??? since Aubin wasn't killed by TWSITD and I think it was mentioned that Rhea and co took the idea of giving out Crests but if it wasn't, we might have the another Yuri case here (And maybe how the Fetters were made). But that begs the question on why didn't Aubin just ask the Divine Soverign for help (maybe he left by this point to Fodlan Maybe Cai was one of the 4 apostles.
It can also be after the War where we know some of the descendants of the Elites ran away from Fodlan and Dietrich is one of them who either on his own or by TWISTD design went to continue to overthrow the Nabateans even after losing to Serios and co.
- Nuked the old world -> Agarthans
-Possible set up unneeded rebellions and overthrow possible peaceful settlement -> Agarthans
- Mass genocide and Nemesis war of Liberation Conquest -> Agarthans
- The War of Eagle and Lion and possible breaking up the Empire as a whole -> Agarthans
- Edelgards whole past -> Agarthans
- Tragedy of Duscar -> Agarthans
At this point I wouldn't be shocked if the initial idea about Crests = Nobles that Nemesis set up and Rhea maintained also came from them