The longer this game goes on, the more I realize that this is all about aetherial balance. Aether is the fabric from which all life is woven, it is the basic unit of reality. When the six elements are in balance, life thrives, and when it is imbalanced, life is threatened and the world is in danger of being torn asunder. Think of it like a canoe shaken to capsizing.
Allow me to propose a new theory:
The prophecies foretold an era of chaotic imbalance representing each of the six elements, but some predicted a seventh as well, later confirmed by the discovery of the Seventh Verse of the Divine Chronicles and then the arrival of the era itself. This cognates exactly with the Seven Hells. Eorzeans believe that the first six hells are each dominated by a single element, but in the seventh resides the lord of the underworld, the Black Warden. This is the force that drives eternal oblivion, which is arguably the exact opposite of the life bestowed by aether.
Now, beyond the light of the crystal (itself composed of aether) lies what is known as the void. Some believe that the voidsent monsters we see in Eorzea are sent by an outside force that seeks to imbalance the realm. This force is likely the Ascians, who we know want to see the realm destroyed and the crystal dark. Posing as Paragons, the saints of the beast tribes, these shadowless entities from beyond the crystal's light encouraged the summoning of primals, which consume the realm's aether and drive the aetherial balance offcenter (thus into an umbral era).
Now, look again at the masked mage. Using the knowledge and power of the Paragons, he drives the beasts of the realm mad by destabilizing the aether and later requests the aid of some greater dark force and uses it to summon voidsent. Now, look back to Urianger Augurelt. When he first came to Eorzea, he lied his cryptic ass off about who he was and what he was doing there so he could act as a diversion while his comrades established the Grand Companies. One of the lies he told was that he was a member of a group of mages that were aligned with the seven demon lords of the seven hells who sought only to plunge the world into chaos.
Starting to add up?
I used to joke about how the Seventh Umbral Era seemed like when we would finally be forced to clean up the mess that every other era has been putting band aids on for thousands of years, but I think that's actually more correct that I anticipated.
This is the umbral era of oblivion and our last stand against the Ascians to save the source of all life, and that masked bastard was the first casualty to the Adversary.