The premise is that it picks up a few years after the events of Darkmage when the protagonist of said book, Darien, traded his soul for that of his beloved. It managed to save the world from the invasion of hell's forces but only for a short while. Worse, Darien now finds himself enslaved by the very people he almost destroyed with his godlike powers. Powers they now desire to turn upon the kingdoms which he formerly protected.
In the hands of any other author, this would be a story about how Darien was either corrupted to the side of evil or cleverly subverts events so he can be freed by his plucky band of heroic friends. Instead, M.L Spencer uses it as an opportunity to dissect the typical "Good vs. Evil" narratives which so many other fantasy novels rely on. J.R.R. Tolkien may have talked about how the people under the control of Sauron were not so different from the men of the West but M.L. Spencer forces our hero to actually get to know them and why they were invading his homeland.
We also get a good look at the forces of "good" in this book and it's not a flattering picture. Normally, the good guys start divided before eventually coming together in order to fight the bad guys off. Here, it's shown the divisions between the kingdoms are far deeper than anyone could have guessed. When on the verge of extinction, we see treason and backstabbing to make sure whoever survives will have uncontested control over the remains.
I especially loved the story of Meiran, Darien's lover who he traded his soul for. In simple terms, the story is how it turned out Darien loved her a lot more than she loved him. Continually, we see him show he's willing to go the extra mile or the extra five hundred miles to try to save everyone. However, Meiran always finds herself balking or trying to justify a reason why Darien's actions were wrong or should be condemned. She can't let go of the "us vs. them" mentality that renders everything on the side of the Darklands to be evil while those who oppose them are good. It makes her initially naive then later just bigoted.
The culture of the Darklands is influenced by what appears to be Middle Eastern and Indian ones but doesn't exoticize them. It's also got heaping doses of Mordor with questions as to how a people could live in a land which is devastated by hellish energy as well as perpetually dark. I especially liked the discussion of "lightbringers" as a literal description of what their job is as well as a joke for Abrahamic mythology. There's a lot of great action in the story and some fairly epic magical battles which work well to keep things exciting between the philosophical discussions between characters.
Fans of the original novel, Darkstorm, will be pleased at the return of Quinn the Artificer. Despite having died millennium ago, he's a character which reality just can't keep down. I loved his many conversations with Meiran as she keeps trying to frame things in terms of good vs. evil while he tears them apart with practiced ease. It reminded me of the conversations between Dragonlance's Raistlin and Crysania. I, honestly, wouldn't be adverse to seeing a relationship between these two characters developing.
In conclusion, this is an excellent fantasy novel which I think fans of the first two will love. M.L. Spencer has created a great morally ambiguous set of stories which challenge a lot of the preconceptions of high fantasy while maintaining many of its tropes. They're also just plain fun books which are setting up to a great finale.