It took me a bit to get in to the story again, as it's been so long since I've read Radiant Shadows. I had to get reaquainted with all of the characters.
Darkest Mercy tied things up nicely. I would have liked to see Devlin and Ani play more of a role in this book than just being mentioned by characters, but you can't have everything can you? I loved how strong Aislinn became in this book. Less of the "oh crap I don't know what I'm doing" and more of the "I don't know what I'm doing but I'll fake it till I make it". She had really begun to irritate me in the other books. Keenan as well. Well, he irritated me from the beginning because of what he did to Ash, but I guess if he hadn't there wouldn't have been a series. So... moving on.
Donia had to have been my favorite in this book. She definitely became a force to be reckoned with (so did another regent, but I don't want to give away too much). And the pact that Keenan made with Innis reminds me of Aragorn and the undead army he recruited in Return of the King.
I am sad that the series ended but I am happy with the way it did. I always hate when a series I love comes to an end. It's like a good friend has moved away. But kudos to you Melissa Marr for writing such a remarkable tale of Faerie.