At the end of the last book, when all hell was raining down, the villain Marina cried out to her former captive Raf that her plan had failed.
In Hellhound King, we find out why the hell she did that. It was a big surprise that the two had been lovers before she imprisoned him in a box for years. Also, surprising that Raf was out for revenge for the murder of his wife, son, and brother a hundred years before that.
Also, surprising, just how many characters went from bad to good then bad again in this book.
Although enjoyable, I'd have to say this was the worst of the series. The others were simply so much better!