So these are the first conceptual images I’ve come up with.
I have been working on the mythology and background a bit more,
to help establish the character a little better.
At first I was going with your standard Demon Overlord, which could be compared to ones like Diablo or the Balrog from "LOTR". (Wings, Horns, ect.)
Now I'm thinking of going a little more abstract.
I'm considering making the Demon formless like fire, light, gas , ect. Until it takes its embodiment, which will be the character to present. Since it comes from another dimension, it's form doesn't take a true shape until it's allowed out of its' confines to activate; then we would recognize it as a life form of proportions.
The Archaea or Archaeons' purpose is to take life back to ground zero, sort of like a power off button for life (organic or artificial alike). It takes shape in the form of a biological plague, that reproduces itself by converting the energy of matter into replications of its "viral self." The "viral appearance" is the construction of the Demonic Character, but they will each posses a small amount of the essence from the actual Overlord (the shapeless mass) which is about the size of a star.
Imagine a glowing light located in their bodies, which is a little part of the demon.
So technically it is all one massive demon who spreads itself by destroying life at its' core on a microscopic level, producing something similar to the "Grey Goo Effect."
Ok, enough of that; everything may change later anyways.
Now its time to post some art.
These are some roughs I was playing with to get some ideas.