This is a hard book to talk about because too many of the core elements of the plot come in via twists partway through, and the back-cover blurb in my edition gives only a bare hint of what's going to happen after page 150. Here's what we do know as the story starts.
At the end of the previous book the seemingly close-knit group of five--Alex Verus, diviner; Luna, his apprentice; Variam, a fire mage; Sonder, a time mage; and Anne Walker, a life mage--has broken up over the ethics of Alex's decisions. Sonder is inducted into the Council Keepers, who maintain order among mages and adepts, and he is quite disgusted with Alex. Anne is strongly against killing or hurting anyone, even in self-defence, and having rejected Alex for that reason is rejected herself by the mage community. On her own and unprotected, Anne is kidnapped and, as the blurb says, is "taken into the shadow realm of Sagash," the shockingly cruel and very powerful Dark Mage who was once her master.
Alex and Anne are the main focus of this story, and what they have in common is that they escaped the clutches of a Dark Mage master--Anne from Sagash and Alex from Richard Drakh, who's even worse than Sagash. What's different is that Alex was recruited by Richard and served him enthusiastically before discovering the truth and trying to escape, whereas Anne was trapped in Sagash's "shadow realm" (something like a different dimension) and forced from the beginning to do horrible things. Both Alex and Anne suffered extreme torture under their masters; Alex escaped at last and Variam risked the shadows to rescue Anne.
The important contribution that this book makes to the Alex Verus series is a major deepening of the characters of Alex and Anne; both of them, but especially Alex, have critical decisions to make, and as you may expect they risk death or worse at every step. What you might not expect are (1) the way that their ethical sense is tested in the decisions they make and (2) Alex's abject fear of Richard Drakh when he "resurfaces" (yes, a back-cover spoiler; there it is).
Throughout the book Alex, as a diviner with no other power than to see possible futures, survives again and again by his wits in both dialogue and combat, and he's up against no fewer than six Dark Mages and apprentices, each with different powers and different agendas--can he get them to work against each other, and how well can he trick mages who outmatch him in, really, everything?
Still strongly recommended, even if favorite characters like Luna, Arachne, and Variam take secondary roles (for which four stars instead of five).