Ghost Exile book 9. And conclusion. spoilers ahead for earlier works
Callastas has what he needs to cast his great spell, which will destroy the world. The army has Sulaman, the prince, and so is no longer the rebellious force, but it still must face the Grand Vizier's. Caina is trapped on Pyramid Island with Morgant and Annarah and entirely too many undead for the three of them to fight.
It involves a necromancer's trophies, a magical elixir that heals with great force, fighting a battle with an alchemist, predicting death, the secret of why the djinni thinks Caina is the one he waits for, opening and jamming open a gate, and more.