The premise of the series is that a ragtag band of heroes tried to save the world a 1000 years ago and failed miserably. Instead, the world has been divided into a dark and depressing hellscape as well a picturesque Gondorian paradise. You'd think this would be an easy set up for a conflict between the "good" people from the pretty lands and the bad people from the pseudo-Mordor, right? Wrong. It's much more like George R.R. Martin's Wildlings versus the North except the Watch equivalent are mages. Except in this universe, Jon Snow defects to join the side of the Wildlings and they really are going to butcher as many people as necessary to.
For the past few books, we've watched the main character of Darien struggle with his loyalties. Darien has resolved that conflict and has now decided to go 100% #TeamDarklands. The problem is that it was never about serving one side absolutely and all he's done is become a monster in service to a new cause versus recognizing the humanity of both sides. However, the other side is #TeamLightlands and the only thing this is going to accomplish is get more people killed.
Darien is a character who has had a magnificent and complicated journey from plucky hero to deranged fanatic. It's wrong to simply term him as a villain, though, even when he's planning on massacring every survivor of the villages he conquers in order to make sure there's enough food for his army of refugees. His decisions are irrational but he's certain he's making the most logical plans he can as both PTSD as well as stress have worn him to the quick.
On the other side of the conflict is Kyel, who has become the mage Darien tried to be but refuses to cross the line and break his Oath of Harmony (no killing with magic). This despite the fact he's leading a massive army to exterminate the Darklanders. Kyel is the more traditionally heroic of the two but he's surrounded by people with absolutely no honor who, ironically, don't trust him because he thinks of the Darklanders as people.
There's a third faction of individuals trying to figure out a way to deal with the "Convergence" that is coming around for a second time and potentially will render the entire war moot. I, personally, like their story most as I was always fond of its characters and don't feel comfortable spoiling all of the twists as well as turns there. It does, however, nicely illustrate the healing power of revenge. Really, what can you say about a story where murdering your closest ally in saving the world is a legitimate means of doing good?
The series end on a bittersweet note which foregoes the nihilism of Joe Abercrombie's First Law Trilogy and is more akin to Mark Lawrence's The Broken Empire. Some measure of redemption is achieved by a few characters while the majority of the cast doesn't make it out of the finale. It's a lot harsher than the Lord of the Rings but, ironically, I think honors its Tolkien inspiration better than most. War is a traumatizing and soul-destroying activity that never fully is recovered from. It remembers Frodo dies of a broken heart (Journey to the West aside).
In conclusion, this is an epic ending to an epic series. Whatever her future works, M.L. Spencer has created something that I believe deserves to be read by fantasy readers throughout the world. The characters are beautifully illustrated, tragic, and well-realized. The action is amazing and full of amazing feats that would make a blockbuster movie. The morality is also ambiguous but makes real and solid points. The world-building is simple but always consistent with revelations about its structure until the last. Read this series. You won't be disappointed.