The best hidden yet most informative book series in the Kiseki series to date: Black History. Only obtainable during a second or later playthroughs of Sen no Kiseki II, the book series chronicles major events of Erebonia and apparently reveals more than the imperial authorities would like you to know. A translation.
This particular writeup will pull from one of my own personal theories, as well as reach into a curious one I had stumbled upon elsewhere. There are interesting aspects to it, so I felt it was most definitely something worth sharing.
This delicious crack theory suggests that everything we think we know about Zemuria may be false. What if everything since the Great Collapse has been the mere execution of a plan that was drafted prior to the Collapse, which served as a catalyst?