The Necromancer is a very long winded book, lacks character development and repeats itself over and over.
The betrayal of Josh isn't very surprising, the introduction of Aoife and her death in the same book, is just a waste of potential and character.
The going back in time thing, to sink Danu Talis is a bit weird, because if the twins are going to have a fight, which will sink the city, how come that no elder remembers them? None of the new introduced elders in the previous books, have recognized the twins, yet they will somehow cause the fall of their ancient capital city?
The interactions between the characters are getting a bit repetitive, and I'd hoped more interactions between Perenelle & Nicholas, since they're a couple, and Perenelle just escaped from a prison full of monsters.
Besides these plot holes and events, I liked the book. It has an interesting turn of evens, and I also like the elder / Greek Titan Prometheus.
4/5.