@Parker.EXE  
…that’s one of Adagio’s daughters, since that apparently wasn’t obvious.
 
The one who went crazy religious and burned her own sister at the stake because she viewed their mother as a demon, and thus is the reason Adagio disliked Christians for several centuries. Not sure why she isn’t in Hell now that I mention it.

I too am disappointed that they seemed underutilized, but two of my personal fan theories were confirmed by their appearance. 1, Adagio spitting “Human scum” under her breath more or less confirms that, while they have lost their powers, they are basically (quite possibly literally) daemons that look down on mortal races. Even if Aria and Sonata bug the shit out of her, they’re the only people Adagio respects to any degree. 2, Whether they have magic or not they won’t become a recurring antagonist not because they’ve seen the error of their ways, but because of the futility of their actions. They’re a group determined to make it their own way, despite everyone in their way. They haven’t changed much, but just enough that a redemption arc could be feasible.