I honestly don’t remember if Milla made it back in the last book or in this one, so we’re going to pretend it’s this one because I do, at least, remember that this book was Milla heavy.
So Milla becomes the War Chief of all Icecarls because…reasons. She makes it back to The Crones, accidentally killing a Shield Maiden in the process, and gets them up to date on the upcoming war. Since she has a shadow spirit now, she’s not allowed back into Icecarl society. But, she’s also kind of the savior of the Icecarls because she alerted them to the upcoming doom. So they make her War Chief, because a large group of nomadic Viking pirates should be led by a teenage girl who, a few pages ago, was ready to off herself.
Anyway, the Icecarls begin preparations to storm the Castle. Meanwhile, Tal goes back to Aenir in order to approach the Empress with the proof of the upcoming war being orchestrated by top levels of her staff. Turns out, the Empress is pretty much to blame for all the problems of today and she released an unbound shadow spirit into the real world in order for him to bring down the veil and join the worlds again (see, Legend of Korra parallels).
Milla begins her assault on the Castle. It goes as you’d expect a fight between Vikings and magicians wielding lazer light shows to go. Tal makes it back from Aenir and our ever growing group makes their way to the violet (seventh) tower to take on this unbound shadow spirit hoping to tear the veil. Oh, and Tal is now the Emporer because why not?