I won't lie: I was a little bit wary going into this. I don't know if it's just my bad luck as a reader, or if there's a problem endemic to YA, but at least four trilogies I've read in recent years have ended with fizzles rather than bangs, even when the first two and a half books have been great. So I am pleased to report: it does not fizzle! After spending two and a half books building up how evil the Duke is, building to a war, and building up Nya's character, the conclusion shines where other books... uh, didn't so much.
It probably isn't fair to do a one-to-one comparison, but without naming any names (ahem) there was a trilogy that concluded last year where an impoverished teenage girl's main goal was to save her little sister, and as she tried to accomplish that she became the symbol of a rebellion against an oppressive government. I bring this up because some of the pitfalls of that particular book are also elements in play in Darkfall, but here they turn out solidly. The stakes of the war are equally deadly here -- terrible things happen, battles are lost, and people die. Some of it is even Nya's fault. But even when bad things are happening, Nya never checks out; and even when she's being used as a figurehead, she never loses her agency. She's the one who decides to play along with questionable leaders, even when things don't feel right, and then she has to deal with the consequences.
In my review of Blue Fire (the middle book in the series), I mentioned that I had a hard time tracking what happened at the climaxes in the first two books. That was actually not the case in this one -- there was a heck of a lot of action happening near the end, but I never lost track of what was going on with Nya's powers and the Duke's weapons.
Aside from improving on what I found to be the previous book's main weakness, the conclusion maintained all of its strengths: the worldbuilding is great, Nya's voice is strong and she's fun to read about, and the story is compelling. The only tiny quibble I had was that I don't think we got to know one of the characters who died very well beforehand, so it didn't have quite the impact it should have. That's an extremely minor complaint, though. The book is a solid five cupcakes, leaving the series as a whole at four and a half.