The fifth October Daye book. Sequel to Late Eclipses.
It opens with Sylvester taking advantage of her new, more fae abilities -- with some backfill on why (for short, Late Eclipses) -- to teach her more swordplay. And to ask Toby to formally take on Quentin as her squire. Even her objection that she's gotten Quentin shot can't keep him from persuading her to do it.
It's when she gets home with Quentin that things get interesting. Because the Luidaeg is there. To collect. Toby had said that she didn't care what the price was for her help with Blind Michael (back in An Artificial Night), and now the Luidaeg is collecting a steep one, because otherwise there will be war: she must protect the children of the Duke and Duchess of Saltmist.
It involves redwoods, a troll trying to talk to rocks, a child's finger being cut off, pixies and bogeys, a choice for Toby's daughter, a mermaid in a wheelchair, someone Toby knew in her misspent years as a runaway, and lots of weapons being taken out of armories.