I think I told you Eilis is called a Rhevendahli (Person from Rhevendahl). Rhevendahl is now part of the Empire, but it used to be a small independant country, that was before the Sorcerer King conquered it. The Rhevendahli were fearsome magic users and are now some of Vaim's most powerful warriors. Now how and why Eilis is connected to all of this...that's the question isn't it?
As for Arn he will be able to hold his own, the girls simply put may use a different sort of magic, technically Arn is still more powerful. 
The situation with Eilis though is different... she isn't supposed to be this powerful... buuut you're going to need to do some detective work of your own in order to find out why. 
As for it being mystical...maybe it being slightly more ritualistic, but what I have planned is far from ancient. (Grins wickedly)