[quote]The elf girl, dressed in rags that only barely gave her modesty[...][/quote]
This is a bit clunky - reads very RP-ish, which isn't what you're going for. At the very least I'd change "gave" to something like "preserved," but I think it's better to put this description elsewhere. It doesn't seem to belong in this paragraph - this paragraph isn't about what Mara looks like, it's about where she is and what she's doing. Remember, your audience needs shockingly little visual information to be immersed in the scene. 


[quote]Casually, as if leading the man to bed, she lifted him over her shoulder and carried him.[/quote]
This comparison is unclear. I'm not sure what you're getting at. Are you trying to invoke a sexual image here or something more like a parent carrying a sleeping child to bed? 


[quote]For the same magical affinity that let her resist magic let her learn to wield it.[/quote]
This isn't a complete sentence (can't start with this version of "for"). It should be linked to the previous sentence somehow. I know we use incomplete sentences for creative effect sometimes but this one doesn't land. 


[quote]her smooth skin was somewhere between a soft tan and a fine marble white[...][/quote]
This seems like kind of a wide range, and at first it made me think she was mottled in colour like a cat or something. I'd recommend just picking a particular colour and sticking with that. 


[quote]The words came out like the silence between words, noticeable by the absence they left in the very air.[/quote]
This is clunky. Having "words" twice so close together is awkward to read, and the simile doesn't make sense since when speaking normally, there are no silences between words. It's also not quite clear if this is dramatic wording or if her words are literally sucking the sound out of the air (up to you if you want to clarify - not [i]necessary[/i]). For a similar dramatic effect, I'd try comparing the words of the chant to something like the space between heartbeats, since that evokes more tension (but that's up to you). 



Typos: "reagant" should be "reagent" and "breath" in the third-last paragraph should be "breathe." Paragraph starting with "And through it all, she had grit her teeth" isn't spaced properly from preceding paragraph. 


These are the things that jumped out at me as I read them at midnight, lol. They're small nitpicks because the prologue in general is pretty good. I've seen published books that read much worse. Not bad!