Briefly read over the contest post and decided to touch on a few more things.

Umbrians, are they good, bad, or something else?

Maybe they could have the personality of collectors. Their ultimate goal being to silently chronicle the happenings of the endlessverse while unseen in the background. Surviving past the splintering of the Endless, and probably even knowing of the technological path that brought them ruin. They see the timeline of the Endless, other civilizations, and also individuals as Songs where their history is like a series of notes. So when speaking to other factions the Umbri could say: “how goes your song traveller?” aka “How have you been doing?” They believe there is power to be found in learning the histories(songs) of other civilizations. Thus it would be their quest to collect information and science on other civilizations, invading their privacy of course. Because of this they are a little sneaky and off-putting to most people they meet.

They would be a religiously focused race, with maybe a pacifist favour as their second political preference.

Or If they are going to be more sinister in nature then maybe they could be more thievery focused?

Be able to steal technologies or silently leech off resources from other civs.

Maybe have some goody's stolen from the Endless that let the do neat things with dust?

Same political orientation except of course no pacifist favor as the bad guys.

I personally like the collector theme the most.

I have some more ideas on the lore: 

They as a people have suffered an ancient Curse where they were turned into the shadowy phantoms they are today. 

Before the phantom form they were an alien race whose society was very orally centered and wasn't that advanced technologically.

It was a very important tradition of there's to preserve history through the verse of song, because of there oral culture.

Then this all changed when one of their leaders performed "The Great Betrayal" selling their people's souls to other worldly powers. (Old Ones?)

The Great Betrayal cursed the Umbri into the form they are today, and the "Great Betrayer" mysteriously disappeared.

While they are still physically present they have become unable to taste, smell, feel, or most importantly to them, hear or speak naturally. 

Becoming voiceless and unable hear sounds drove the Umbri insane in their early days suffered under their curse. 

They call this period of time the "age of darkness and silence". 

From this age a religous fervor takes hold of the Umbri where they have a dichotomy of Light = Sound/living and Darkness = Silence/Death

They make it their mission to bring back the worldly senses they lost to their curse, to "resore the light and song of their people" if you will. 

The Umbri begin to slowly adapt into a society that uses written languages since they don't really have any other way to communicate.

It's at this time that the Endless come onto the scene.

Once visited by the Endless they are able to greatly expand their scientific knowledge, and eventually develop exactly what they have been looking for, voice modulators.

So they use these voice modulators to speak and to hear sounds (Very well, like a bat's level of hearing). 

These voice modulators float around the Umbri in the shape of Hi-Tech Candlesticks. 

The Candles will flicker whenever an Umbri speaks. 

Very Important Umbri like political Leaders will have multiple of these Candles hovering around them. (To help project their voices)

The Umbri take on a habit of observing and recording the Endless and other Civilizations, fascinated by the sounds they make and the sounds of their cultures.

They are a bit messed up in the head though and tend to be VERY intrusive in their observations, 

often eavesdropping classified information from the various Endless factions but never really understanding the information.

So now they are focused on two main goals: record the "Song" (History and Information) of other Civilizations, 

and bringing back their bodies, be it through reversing their curse or by "observing" enough technology to supplement their incorporeal forms.

I Want to Imagine them as are slightly creepy dark angelical wraiths who have obviously been touched by some other worldly, maybe lovecraftian, force.

Singing hymns that are pleasant but have a slightly off putting feel to them.

(Well most of the faction's core mechanic's have already been designed, but I geuss I'll put this out there anyways.)

Also as a game mechanic, I’d really like it if they suffered from “The curse of Silence” where all of their influence area of planets is covered in a fog that slowly covers their territory like the fog from unexplored space, but maybe textured slightly differently and it regenerates after you explore it.

You would only be able to see into the fog with probes, flying ships into it, or maybe through the new spy mechanic. After a probe or ship leaves with it’s field of vision, then the area explored slowly reverts to the "Silence of the Fog". The only thing left visible in the fog is the name of explored nodes, systems, and the planets inside them. Population and other civ based stats become shrouded. Ships traveling within are also shrouded of course

Even Star Lanes would become shrouded by the fog.

It would make Invading the territory of the Umbri more exciting and challenging.

You could even make it so that the Umbri suffer from it as well being unable to see what goes in and out of their own territory in the early game. 

Then, at some point in the faction quest line, there’s a reward you can get that negates the fog for you. Maybe an empire tech or unique building allowing you to see through the fog while other factions cannot. Turning the fog into a defensive benefit.