Having moved on from the apparently non-existing prototype Doors in Sora no Kiseki The 3rd, I dropped by on my favourite Japanese page full of crackpot theories and ran into the following one, which is actually supported by an in-game quote.
In order to restore the Blaze Demon Castle to its former glory, Anguis No. 2 had to sing a song which contains the following lyrics:
He who controls the threshold of the darkening sky, the crimson emperor wielding a thousand weapons…
Descendant of the guardian of flames, I bestow upon thee this hymn.
If descendant refers to Cedric, this would imply that the Arnor family has been governing the Sept-Terrion of Fire.
Also, the theorist suggests, what if Mariabel was invited to Ouroboros for her ability to extract Sept-Terrion from, say, Rean’s chest?