I didn't like Nick all that much in the last book. I understand him being upset/pissed that Isadora is with his brother and not him, since they both only get that one soul mate since they are twins (which really doesn't work, since they have different fathers, meaning they can't be identical twins - same egg, same soul. Sharing a womb shouldn't have anything to do with soul mates, but I'll go with it anyway.) He acted like a complete ass to Isadora. Really, I wanted her to punch him or something. Then he saves her soul by offering himself to Hades instead. Couldn't you just let me hate you for a while, Nick?
Yeah, I know all of that was about the last book and how I kind of hated Nick then, but it's important. I really don't like reading a book when I'm mad at one of the main characters, since they start out with negative points and have to redeem themselves.
That said, this book jumps straight into the action six months after Nick trades himself for Isadora. I was right in my guess that Cynna, Zagreus' Master Torturer, would end up with Nick. She really gets called that (or something like it, at least) in the book at one point. Nothing is as it seems in Zagreus' dungeons and Cynna is no exception. We actually get Cynna's POV before we get Nick's, which was a little odd.
Nick and Cynna have a very odd relationship. They've known each other for six months, but they don't really know anything about each other. Cynna has ordered Nick tortured. Zagreus wants him broken and he hasn't broken in the six months he's been there. They're running out of time, since Hades is getting impatient. I'm not going into all the finer plot points, since I don't want to really give anything away.
Cynna and Nick escape, but they aren't really free. They go to want is left of the Misos colony that Nick was leader of. They get a little time together before the next crisis pops up - Isadora is sick and they think it's the soul mate bond to Nick. I'll admit that I expected something like that to happen. There's a lot of drama with that. I'll also admit that I didn't like Isadora all that much in this one. I think it was Cynna's dislike of Isadora coloring my view of her. I've liked her in all the other books.
The book ends with a battle between several armies and the Argoleans, which includes Cynna giving herself to Zagreus in exchange for him calling his army off and Nick making a deal with Zeus and Poseidon. You'd think they'd learn at some point to not make deals with Gods!
There was something very dark about the relationship between Nick and Cynna. Maybe it's just from how it started - knowing each other as prisoner and torturer. They both seem to have a thing for (as Orpheus puts it) whips-and-chains. There are no whips, but there seem to be a lot of chains. I don't really have an issue with it normally, but there was something really twisted about it since she'd tortured him at one point (on Zagreus' orders, but still). It works for them, but I expected a lot of jokes at their expense.