Hmm if i'm going to come up with a theory then it would be this.
There exists a rouge archon (Fenrir / Diablos), this archon was sealed off in the city of Lucis by the other archons, the crystal that is housed in Lucis was used to seal this rouge Archon, thus the royalty of the Cealum bloodline are entrusted with maintaining the seal and are granted ring magic by the three giants in the dawn trailer for doing so.
The stronger the seal, the stronger the crystal, the more powerful the magic that the kin o Cealum can wield from the ring of lucis.
The plague of the stars is a side effect of the seal becoming weaker, the need for King Regis to use his life force to maintain this seal is because the crystal can't sustain the pressure from both Niflheims invasion and this rouge archon.
Niflheim views this roug as a god and worships it, they wish to free this rouge archon from its seal in Lucis.
The seal is multi layered, in order for it to undone the bearer of the ring of Lucis must be dead, then the other archons must be killed as well.
The archons (all of them accept the rouge one) are guardians of EOS while the rouge Archon wishes to destroy it.
once the wielder of the ring of lucis is no longer of this world and the archons salin this rouge archon breaks free and sends the world of EOS into an eternal night, this rouge archon would be Diablos.
There is a secret archon who is the archon of the sun / moon or space ( your pick really ) who only appears when the world of EOS is in grave danger of dying this archon is Bahamut.
as the other archons are dead, its up to noctis and to wield the might of Bahamut the last archon to take down Diablos who is revered by Niflheim.
The knights of round aren't a normal archon because they can't manifest themselves in a physical form the reason for this is because each one of the XIII knights has handed their weapons to the kin of Cealum, in order for them to be able to manifest themselves a kin of Cealum must gather their weapons in order to summon them.
The sword of Arthur is currently held by King Regis, without all XIII of them it is impossible for the knights of round to manifest themselves in a physical form. Knowing this Regis sends his son to gather the relics of the knights of round and inherit the pact between the kin of Cealum and the Knights of round.
They are the invisible beings we see in the dawn trailer.
I believe that the order for which weapons we will get when are the same as the summon order of the Knights of round in FF.
Once all 13 of the weapons of the knights of round are gathered and Noct acquired the ring of Lucis he can summon the invisible beings we have seen in the dawn trailer in their physical form and reclaim the kingdom of Lucis from Niflheim and Diablos.
The reason i think this is because of the bellow on the Knights of round (SPOILERS FOR FF TYPE 0 HD AND THE NOVEL!!!!! You've been WARNED)