Theory is mostly broken, but one part has been proven thanks to Sen no Kiseki III. Details of the break/proven are behind the cut, due to spoilers.
As of Sen II, players have dealt with two of them, know the location of two other, and have three more to look forward to. A brief theory regarding the unrevealed Sept-Terrion.
Partially Proven Theory Notes (Added by omgfloofy) (11/3/2017):
The Sept-Terrion in Erebonia are of Fire and Earth. As Fire is mentioned later on, this theory is stamped as ‘Partially Proven,’ but water and earth were incorrect.
This theory should be revisited in the future, regarding the other Sept-Terrion, and this note will be updated accordingly regarding the remaining Sept-Terrion.
Though introduced relatively late in Ao no Kiseki, the Master Arts came with something that was way more helpful than the actual arts themselves: the cut-in illustrations used during those Master Arts. Apart from gorgeously designed, it is commonly believed that these illustration depict the Sept-Terrion. This theory will discuss the two that have been found in Sora no Kiseki SC and Ao no Kiseki before theorising the possible origins of the other five.
The Master Arts illustration are sorted like the elements they represent – Earth, Water, Fire, Wind, Time, Space and Mirage – and at present we can familiarise ourselves with the two on the far right: the Sept-Terrions for Space and Mirage, better known as the Aureole found in Liberl and the Demiurgos recreated by that family in Crossbell. We also see their two guardians, the ancient dragon Regnard and ancient wolf Zeit (which actually is German for ‘time’), respectively.
Dominion No. 2 revealed that there are not one, but two Sept-Terrion within Erebonian borders. If the lore is to be believed, the Erebonian predecessors received one from Eidos. And, perhaps without knowing, obtained another one through the annexations of neighbouring countries, such as Jurai as discussed in Sen II. Another explanation about how Erebonia ended up with two Sept-Terrion is that they always had two and represent the two fighting deities as chronicled in the Dark Ages books by Dominion No. 2 himself. Rather than guessing which Sept-Terrion lay hidden within Erebonia, let us move on toward the others.
The Sept-Terrion of Time
The Master Arts illustration for the Sept-Terrion of Time depicts an old bearded man, sitting on top of a clock that reads five past ten. On the face of the clock, we find the depiction of a star. This theory argues that this Sept-Terrion is kept within the Holy Nation of Alteria, home of the Septian Church. As argued by Floofy before, “the symbol of the Grals Ritter is very much defined by the star […]”. The prominent star symbol in this illustration is surrounded by what looks like a clock. Keep in mind that, like a clock, the Dominion will never consist of more than twelve members at once.
If we push this analogy even further, we can interpret the bearded man as the personification of the Septian Church or perhaps its absolute leader (a Pope?), which serves as the guardian for a Sept-Terrion held in Alteria. Though organisations within the Church, we know that the interests of the Grals Ritter and Dominion and not always perfectly aligned with the Septian Church as a whole. This illustration may represent that.
The Sept-Terrion of Wind
Though we not as much about the nation of Leman as we would like to, their leading role in the development of Orbal technologies led me to believe that the Master Arts illustration for the Sept-Terrion of Wind represents theirs. While the evidence is limited, we may interpret the enormous eye depicted in the illustration as Leman’s theoretical omnipresence in Zemuria.
Taking into consideration what they have accomplished in Orbal technology since the Epstein Foundation was established, it is not hard to imagine the Epstein Foundation is tasked with securing one of the Sept-Terrion on behalf of their government.
The Sept-Terrion of Earth
On the far left, we find the Master Arts illustration for the Sept-Terrion of Earth. It stars two tortoises stacked on top of one another, of which the one on top depicts the recurring mythological trope of the world turtle, which was said to carry the world on its back. Though excelling in strength, our beloved Calvardian martial artist Jin (Zane) comes with the affinity of Earth as he joins the party throughout the Sora titles. There’s undoubtedly an analogy with the Chinese legendary tortoise Genbu before us, apart from Jin mentioning this between the lines.
The Sept-Terrion of Water and Fire
That leaves the two Erebonian Sept-Terrion to the elements of Water and Fire. Though we have little proof to go by right now (apart from Erebonia’s fiery attitude to take over the world), there are some interesting observations to be made here. First of all, Erebonia’s capital Heimdallr is called ‘the Crimson Imperial City’, built around the governmental palace called ‘Valflame’. The imperial family, clothed in red and surrounded by red flowers, are descendants of emperor Dreichels. On the plaza in front of the Valflame we are introduced to a statue of Dreichels, sporting a beard and crown, resting one hand on his majestic sword, just like the crowned bearded man depicted on the Master Arts illustration.
Whoever is depicted in the illustration for the Sept-Terrion of Water will continue to be shrouded in mystery. One might say that she represents Lianne Sandlot, Dreichels’ partner during the 10th Century Lion War as they are both blonde and, growing up in the vicinity of Legram and the Lohengrin Castle, share a certain affinity with water. Though, with Sen and Sen II continuously hinting that Lianne Sandlot and Arianhrod are the same person despite the 250-ish year gap, we cannot simply assume at this stage this card to depict Sandlot. In any case, there is a nifty little contrast at play in between the Master Arts illustrations for Water and Fire: woman/man, pouring (down)/holding (up), and, of course, water and fire.
Taking the developments of Sen II into consideration, we might end up getting an “A Song of Water and Fire” next year.