I do have two questions. How advanced are the elven villages in comparison to the human nations?
Secondly, would this fit in with the magic.
"All things fall into shadow, even light. Shadows do not represent the absence of light, they show the presence of darkness. Thus, shadow and darkness are not death or diminishment; they are the fundamental state of the universe, the constant that existed before, that exists now, and that will exist when all other things are snuffed out. Just as magic has an opposite. An ancient formless substance that fills the space between matter.
The shadowcaster understands the true, primal power of darkness, attunes herself to the Plane of Shadow, and learns great shadow mysteries the equal of any mundane spell. These dark casters are workers of alien magic, possessing an occult understanding of the world and magic that even other spellcasters find disturbing."
I like the direction you're taking shadow magic, and I like the way you explain it. The only thing I don't want to allow is the introduction of actual "aliens". However, this doesn't mean you have to change anything. I just want to point out your usage of the word "alien" would not be in reference to an actual aliens race, but rather, it's meant as "foreign" or "strange".
It also makes me think we're talking about dark matter manipulation, which actually already exists on a planet wide scale, in the form of gravity manipulation, the primary way hover cars and flying ships work.
This brings me back to your first question, "How technologically advanced are the elven races." The answer to that varies on which race we're talking about. For example, the Volcano Elves, living on the isolated continent of Numgam, are very primitive, and the City Elves, living among humans, are just as technologically advanced as the humans themselves.
Then, above that, we would actually have the Cave Elves. Their village would be more like an underground city that heavily relies on dark matter and gravity manipulation. And their main export would be dark matter itself, a highly sought after resource. In other words, the Cave Elves would actually be the MOST technologically advanced civilization on the planet, but this would only be known to them and a few higher ranking individuals in the Mutrenu continent.