@Tracy–I like it! Although I do have a couple suggestions: I think you can afford to cut out the phrase “something resembling a” (we understand from context that it’s not literally turning into a model), and I’d find a more specific term than “children’s toy model”, if you can.
@Jenn–I’ll add my voice to the chorus and say that I really like yours!
@Heidi–That’s pretty amusing; I can see this being a fun narrator. 🙂 One quick note, though: “summersault” is actually spelled “somersault”.
@nefaerious–I love this kind of mysterious stuff in a prologue, so props for that! I think you can also probably afford to trim this sentence down a bit. For example, I would cut the word “umbral”. Don’t get me wrong–it’s a great word! That said, in writing for YA, it has the ring of an SAT word, and the last thing you want to do is send teen readers scrambling for their dictionaries with your first line (or worse, make them put down the book). I would also tighten “wall of obsidian” into “obsidian wall” if the wall is literally made from volcanic glass. Additionally, I’d cut the phrase “jutting up behind it” because it isn’t immediately clear whether the referent of “it” is the figure or the wall (and because the phrase doesn’t contribute much to the sentence). Hope that helps!
@laura–It’s an intriguing opening that sparks a lot of questions for the reader right off the bat. 🙂 I do think you could make it more vivid if you picked a verb other than “seemed” (just because it’s your first line and all). I don’t know anything about the situation and characters, so I’m not sure I can suggest an alternative that would feel appropriate to you, but that’s just my two cents. 🙂