Dark is a bit light. Perhaps pitch would be a better description of the tone throughout. Actually made The Empire Strikes Back look like Pollyanna by the end.
We center mainly on Hal and Hunter, the last two members of the Brothers and Sisters of Dragons of the current pentad following The Age of Misrule. Hal is an inofficious bureaucrat with the remains of the government (currently seated at Oxford) and Hunter is something of a Soldier of Fortune currently employed by the government. Both come to find out about their position around the midpoint of the book. Mallory and Sophie from Book 1 show up when Hunter is sent to capture of of the Brothers of Dragons on behalf of the government. Sophie winds up supposedly dead and in Lugh's court thanks to Cerridwyn's intervention. Where she meets a powerless Caitlin from Book 2, who in turn becomes an avatar of The Morrigan again.
Any rate, we get much more on The Void, the Anti-Life, that noticed the rise of humanity after the defeat of Balor during the Battle of London. We meet street gangs going around with Red V's on their chests who think of Ryan Veitch, the Great Betrayer, as some kind of Messiah. (Which, given his role in the next book...) Mind you, when Shavi, Laura, and Ruth show up about 2/3 of the way through, they still think of Ryan as a good man who had the misfortune of being used by the gods.
I'm getting ahead of myself a bit here. Hunter more or less plays the role of tactician of the new 5, trying very hard to get everyone in the right place. Hal, in the meantime, has a more cerebral task, tracking down hints of Avalon in a Poussin painting. (And here you though Poussin only painted triangles disguised as historical figures.)
Said painting, complete with Dan Brown style anagrams.
And then we see how the present of the next trilogy begins.
As I said at the outset, this is a very dark book, with very few and very faint glimmers of hope lighting the last days of humanity. And the last days of the Golden Ones, really. I mean, everything changes at the end.
Yeah. I think I'll return to the final trilogy sooner than later, since I'm kind of curious how this all will turn out.