The Throne of Glass series is part of the new-adult fantasy genre, a subdivision of young adult that features protagonists in their late teens or early 20s and includes increasingly explicit depictions of sexuality and violence. In her work, Sarah J. Maas draws on a variety of sources. In The Kingdom of Ash, for example, the Fae, Queen Mab, and the Little Folk come from Celtic mythology; the depiction of witches from Eastern European folklore; and the Southern Continent’s khaganate structure from the nomadic Turkic and Mongol societies of the medieval period.

The Kingdom of Ash is the last novel of the series; as such, it describes the final confrontation between the forces of good and evil that have been amassing in the other novels. A brief synopsis of events leading up to this final war follows. Aelin’s world is plagued by parasitic demons called the Valg, who hail from an unnamed home dimension and use Wyrdkeys to travel between worlds through Wyrdgates. Two Valg Kings—Orcus and Mantyx—have long since been banished from the world by Maeve—a Valg Queen disguised as Fae—but the final Valg King, Erawan, remains. Throughout the series, Erawan secretly possesses Duke Perrington of Adarlan as he searches for the Wyrdkeys to bring back his brothers, find Maeve, and conquer the world.