With the process of how this faction came about. They are ancient compared to all the factions, predating even the Endless. And so the idea for the name comes from the times of antiquitity. These are shadows, and so the idea fell to me that the mythology of Greek, Roman, Egyptian, Norse, etc cultures have something on shades/ghosts/spirits. 

To me the Greek heading of shades fits nicely, as well as the basis for them, as well as that of the Latin cultural similarity.

σκιά - skia (/ski.ǎː/) translates to shadow, but is also used for shade, and by shade I mean the spirits of the dead floating about through Hades. Of course I am no native Greek and am relying on a couple books and wiki entries for my thoughts.

In Latin the term would be Umbra, but that always seems a bit more used than the former term.

Then I looked to Latin mythology for something similar looking for a neutral term since there is a name for good ghosts and bad ghosts. The term Manes is a term for ancestral spirits, which has two other versions as mentioned, "di Inferi" for " the gods below," and "di superi" for "the gods above." But I would like to try and keep the name a bit more neutral so I simply stuck with Manes.

Rökkr is from Norse mythology and means shadow. The Rökkr can be seen as the darkness that consumes the gods of Asgard, they are more accurately seen as the shadow: causal emanations of the acausal darkness. 

But yeah, that's just the thought process I went through. Something latin, something greek, something norse. It seemed like a nice mix.

This is great! Love the blending of various ancient cultures, but im wondering where 'manes' comes into it?

I didn't really use Manes, I felt like mixing in too many of the names would make seem like I simply hit the keyboard with my face, hah. So I stuck with something latin by using the word Obscura rather than Manes.