This is the third book of a trilogy a la The Lord of the Rings -- a book chopped into three parts. I reviewed the first two here. And there will be SPOILERS ahead.
It opens with a foursome of Eramus, Mab, Gregor, and Miranda herself tramping through a hellish swamp in search of the other siblings that the Hellwinds blew away, in hopes of collecting them all in time to rescue their father from the Queen of Air and Darkness, who intends to sacrifice him on Twelfth Night.
With Eramus, of course, blaming it all on Miranda.
They unexpectedly meet an old enemy who is not quite a damned soul any more, but trying to win free by helping others win free. He gives them some help in getting through hell, but they meet many more perils. And problems. And discoveries. The question of who is Miranda's mother gets more and more twisted. There are not so many revelations as in the first two, partly because in the course of descending through Hell, they have to work out the implications of the past revelations. And fix what can be fixed and stop what can be stopped. And learn more about the structure of the universe and what makes it run. And face many, many, many temptations.
A fine novel providing a rousing conclusion.