Onyx Storm is a fantasy novel written by Rebecca Yarros and published by Red Tower Books.

[4] The first book, Fourth Wing, chronicles the protagonist Violet's first year at the fictional Basgiath War College, where she becomes a dragon rider.

Meanwhile, the iron squad visits Sawyer, who is adjusting to life as an amputee, while Lynx and Aaric's failure to manifest signets raises concerns.

Knowing that Andarna, Violet's second dragon, is of the seventh dragon breed needed to power the wardstone in Aretia, Violet is sent on a quest to find the missing breed but must first fulfill her deal with Viscount Tecarus safely deliver the fliers to Basgiath for unlimited access to his library.

Just as the Senarium moves to dismiss the fliers, Violet's squad nullifies the rune blocking their channeling, salvaging the truce with Poromiel.

General Aetos accuses Violet of treason, but a royal missive preemptively pardons all dragon riders who went to Aretia and reinstates Xaden as Duke of Tyrrendor.

When a nearby village is attacked, they rush into battle, uncovering shocking truths: Garrick has a second signet, Theophanie the silver-haired venin is a Maven who wants to mentor Violet when she turns, and Andarna is an irid.

While Xaden is away, Violet sharpens her skills and repeatedly visits Jack Barlowe, searching for answers only to find none.

She enlists Dain to steal it from his father's office, and he delivers, leaving her with pages of secrets and a riddle to decode.

After a brief scuffle, Halden delivers a message from Tecarus, arranging a meeting with King Courtlyn of Deverelli in exchange for a powerful relic lost in venin-infested Anca.

The new squad consists of Ridoc, Xaden, Dain, Mira, Maren, Tragen, Cat, her cousin Drake, and unfortunately Halden and his guard.

Xaden shows up in Basgiath and Violet notices the flecks in his eyes are no longer golden but permanently amber, a direct result of his channeling.

Aaric, defying orders and risking court-martial, replaces Halden as the royal representative, officially launching the quest.

The squad returns to Basgiath with a heartbroken Andarna and shattered hopes, only to learn Queen Maraya has been killed and Tecarus has crowned himself King of Poromiel, making Cat third in line for the throne.

When the squad with Sawyer, follows for their rotation to Aretia, Violet expects a broken man but finds Xaden and Brennan still fighting for a cure.

When Theophanie shows up, Violet defends the priestesses, but luckily when an irid arrives, firing up the Aretia wards the venin vanish.

[11] In the first week following its publication, Onyx Storm sold 2.7 million copies, surpassing any other adult novel in the previous 20 years according to the data provider BookScan.

[12] Kelsey Hall of BookTrib called Onyx Storm "a thrilling and emotionally gripping fantasy" and praised the worldbuilding, relationships between characters, and storytelling.

[14] Laura Hackett of The Times criticized the book, saying that she did not enjoy Iron Flame and Onyx Storm as much as Fourth Wing.

She said Onyx Storm "removes the warm familiarity of school and replaces it with more and more outlandish dangers and complex magical terminology", and described the romantic elements as cringeworthy.