A Court of Wings and Ruin

Feyre Archeron enters this book seemingly broken and back under Tamlin's control at the Spring Court, but it's a deliberate ruse: she is secretly working for Rhysand, High Lord of the Night Court, and their Inner Circle to spy on Tamlin's dealings with the invading King of Hybern. Once her cover is blown, she escapes back to the Night Court, and it's revealed that her apparent estrangement from Rhysand was staged the whole time. Rhysand then does something unprecedented and names Feyre High Lady of the Night Court, making her his full political equal, not just his mate.

Much of the book follows Feyre traveling with members of the Inner Circle (Rhysand, Cassian, Azriel, Mor, Amren) to the other courts of Prythian, trying to convince the other High Lords to set aside their rivalries and unite against King Hybern, who intends to use the Cauldron to tear down the wall between the human lands and Prythian and conquer both. Feyre's sisters, Nesta and Elain, are pulled into this conflict directly and end up forcibly transformed into High Fae by the Cauldron against their will, a trauma both are still reeling from by the book's end. Elain is also discovered to share a mating bond with Lucien, Tamlin's former emissary, though it's an unwanted, unresolved bond for her.

Tamlin allies with Hybern out of desperation and manipulation but eventually turns against the King once he sees the true scope of the coming slaughter. In the final battle at the human lands, Rhysand is killed protecting the others and is brought back to life when the assembled High Lords each contribute part of their own power to remake him. Amren, revealed over the course of the book to be an ancient primordial being rather than true High Fae, sacrifices her original nature and power to help win the fight, permanently changing what she is. Jurian, long seemingly allied with the human queens serving Hybern, switches sides at the last moment. The King of Hybern is finally killed when Nesta, drawing on her strange new connection to the Cauldron, commands it to destroy him.

By the book's end, Hybern's invasion is broken, the wall between human and Fae lands is dismantled, and Tamlin survives but is left weakened and diminished, spared at Feyre's insistence. Feyre and Rhysand formalize their union, with Feyre firmly established as High Lady. Going into the next book, several threads are left open: the fallout and adjustment to the wall's removal and a wall-less world shared by humans and Fae; the unresolved, charged tension between Cassian and Nesta, who is struggling with trauma and her forced transformation; the strained, unspoken feelings between Mor and Azriel, complicated by Mor's history with Eris of the Autumn Court; Elain's unresolved mating bond with Lucien; and the broader political rebuilding of Prythian's courts and their new, fragile alliance under Feyre and Rhysand's leadership.