WARNING! CONSPIRATION THEORY BELOW!!!

I suppose that those four-eyed creatures were much much older and more advanced that the Endless, and THEY had created the Dust, whereas Endless just re-discovered it (In the Vision trailer, the narrator says that spreading the Dust was their plan). Furthermore, we know that Dust can change personality, or even the whole way you think, and those ,,Ancients'' referred to themselves as gods; so maybe the Dust was a tool to enslave the lesser races and keep them under control. Eons after the disappearance of the Ancients, the Endless start using Dust and soon then wage the Dust War - maybe some Ancients survived in the sleeping Dust and THEY caused this war in order to eliminate their competitor. Next, the Crystal from DotE. Surely linked with the Dust. As we know from the first trailer of ES2, it was on the board of the Grey Owl after the departure from Auriga. The narrator of this trailer is very very old creature, as we can suppose from what he says. And the only so ancient being was the Crystal, so maybe it held the consciousness of the Ancient. Lastly, the ending of the Vision trailers is different for Sophons and Vodyani - there is a circle of stars after disappearing of the face in the Sophon trailer, and there is not in Vodyani one. And the words about ,,those who stand against us"... I suppose that now they are awakening, and some of the races will support them, while the others will fight with them. This also fits the slogan in the end: Sophons have ,,Your vision. Their future" which means that our decisions will impact the fate of the Ancients, whereas Vodyani have ,,Their vision. Your future" which means the opposite - the Ancients' decisions will impact the fate of us. I think that the Tabernacle of Lies from Vodyani questline may keep the explanation.

This is a stretch, but he could be Icarael.

From the fan wiki.

To [Bigeli] came a ragged fleet of dilapidated ships led by a visionary known as Icarael, who had through sheer force of will and character saved an enormous population of Eaür from death. He had chided, cajoled, raged, and begged them all the way across a hostile universe, hoping for a kinder fate. While the sciences of the Bilgeli stations brought solace to many of the wanderers, Icarael himself was lost to the most ancient of magics; he was struck by the wisdom and the passion of Spotora, and within a week of their arrival in the system he had abdicated his post, left the fleet, and sworn to follow her to the end of time. 

Spotora was less than enchanted by this. She needed no passions other than her work, and the constant flow of refugees entreating Icarael to return to his position did not improve things. Rejection followed rejection until Icarael could no longer contain himself; he committed suicide in her laboratory one day by ordering the Dust to reconfigure her medical equipment, remove his heart, and place it in stasis. There she saw it when she returned to her studies after the next sleep cycle, spinning slowly in the gravity-less environment, paused between beats for all eternity. Curious at last to learn more of this madman, she analyzed the DNA that had created such a mind and such a spirit. It turned out to be the final link in her studies, the last element she required to unlink the conscious from the mortal form. 

While we don't know much of Endless biology let alone Eaür, this was a point after transhumanism had won in the cultural landscape, so it's entirely plausible that Icareal could have made himself a "few" modifications. More importantly, how else could have Spotora have learned to transfer the soul of a sentient being into Dust, than to have observed such an ascenscion in action? Icareal would have predated the first wave of uploads, so he would certainly fit the bill as an "ancient demon" to a race that sees the Virtuals as gods.

Over the time, he would have calmed down, to see his end be the beginning of her glory before the dust wars disrupted any peace he found over the years. Unfortunately, we have no clue what his plans are, beyond a sense of domination.