Which one was this? Ummmm....
Oh yeah, Cade Gets Laid.
Rydstrom Woede lost his unexpectedly inherited demon kingdom to Omort. Omort, in case the name isn't a total gimme, can't be killed. You can behead him, chop him up and burn the pieces and he'll _still_ come back. Omort is a Sorceri, which is sort of like a witch, but not. Rydstrom blames his brother Cadeon, aka, Cade, for losing the kingdom, because when Rydstrom had to go fight some vampires who had attacked the kingdom he summoned Cade to defend the castle and Cade, who had spend most of his life being raised by a foster family, declined in favor of protecting his foster family. Unfortunately, while evading troops trying to make him do his duty, his foster family was offed by revenants. Sucks to be a Rage Demon King or second in line to the throne, apparently.
The disinherited and formerly separated demons are now mostly inseparable, recruiting Lorekind to their cause and presenting a united front to everyone else while privately constantly running each other down and beating the snot out of each other. Nix tells Rydstrom he has one last shot at getting his kingdom back: get a sword from Groot the Metallurgist that will really kill Omort (Groot is a half-brother of Omort, but doesn't like him anyway). A scavenger hunt is set up to get the Woedes to Groot's hideaway and thus the sword.
But Nix isn't the only seer around and Omort's half-sister Sabine (Queen of Illusions) intercepts Rydstrom on her way to meet Cade. And we won't see him or them until the end of the book, and then only as a tantalizing intro to What Happened There, which will be revealed in Book 6, _Kiss of the Demon King_.
Where was I? Oh, yeah. You know, there's an awful lot of plot in these things. Meanwhile, obsessive compulsive thought-to-be-mortal daughter of a Valkyrie mother and a civil engineer father, math teacher and doctoral candidate Holly Ashwin (groan) has been sensing a benevolent presence watching over her lately. Then one night, it doesn't feel benevolent, but it's still a presence and the next thing, she's about to be impregnated against her will be 1 or more of 12 members of the order of Demonaeus because she's The Vessel. The Vessel's kid will be a powerful All Good or All Evil (depending on Dad's nature) who will have a big influence at the Accession -- the last 6 of 7 of these have been Evil, which does not bode well for Ms. Ashwin. But she gets cranky, her grandparents hit her (well, someone does) with lightning, and she goes all Valkyrie on that demon ass and kills them all. Then she hides in a corner and cries.
Where Cade finds her, he of the benevolent presence, who is about to take her on that scavenger hunt because the deal is: trade Vessel for Sword. And Cade has promised to do this even if the Vessel is his fated mate (cause you know she is) because he doesn't want to disappoint Rydstrom Yet Again (little does he know...wait, that's from the next book).
The scavenger hunt, as Nix knew, would be one glorious almost-got-it-on after another, until they really do (oh, and they're being watched by Groot when they do. Ewwww.). Most of the tension on Holly's part is, but I have a boyfriend (that I'm not actually having sex with) and I don't _want_ to be an Immortal! I hate change! On Cade's part, it's, geez, do I _really_ have to give her up? Awwwwww... Complete with dialogue that could have been ripped from an X-rated [ETA: Rethinking this. More like PG, or PG-13] version of _It Happened One Night_, this is all actually somewhat fun.
And then, it gets a little darker, because Cade really does turn her over to Groot. Yes, it's a romance. Yes, you'll be getting that Happily Ever After. But Ms. Ashwin needs to get a chance to show more of her Valkyrie nature than liking pretty clothes and sexy boots, getting drunk with demons and liking very athletic, frequent sex. Specifically, those 12 Demons from the beginning of the book need some company on the way to wherever dead demons go. She escapes Groot, jumps off a Really High Cliff, picks her broken self up and then kills all the Wendigo for miles around. Which is good, because in the meantime, Cade has to recover from being poisoned, toss Groot into his own furnace, get blown up with the rest of the castle, and then walk out of that mess.
Nix to the rescue, so Holly Ashwin (who is, in fact, knocked up by now) visits Val Hall and mopes around for a while, to be escorted to a home suitable for rearing a child (or children) (which Val Hall is not, because of lightning, if nothing else -- and nothing else would not be accurate) by Nix, who abandons her to the person who bought her the estate, a certain Rage Demon named...
Chopping through all those details, what do we have here? We have screw-up brother Gets It Together and is rewarded with The Sword, the death of a Big Bad, the Girl and soon-to-be-a-Father. We have perfect-brother Fucks Up Horribly. That's a classic plot all by itself -- the fuckup and the perfect siblings trading places in a time of crisis. Then we have the uptight bluestocking turns out to be a words-fail in bed. There is banter. There is discovering-bio-parentage-and-family. Really, it's layered cliche after layered cliche.
But that many layers and it tastes like cake.