Onyx Storm (The Empyrean Book 3) by Rebecca Yarros

After nearly eighteen months at Basgiath War College, Violet Sorrengail knows there’s no more time for lessons or hesitation.

The battle has begun, and with enemies closing in from both outside their walls and within their ranks, trust is in short supply.

Description

Violet must venture beyond the crumbling Aretian wards to seek new allies from unfamiliar lands to stand with Navarre. The journey will test her every skill—her wit, her luck, and her strength—but she will do whatever it takes to protect what she holds dear: her dragons, her family, her home, and him.

Even if it means guarding a secret so dangerous, it could unravel everything.

They need an army, power, magic—and the one thing only Violet can provide: the truth.

But a storm is coming, and not everyone will make it through unscathed.

The Empyrean series is best enjoyed in order.
Reading Order:
Book #1 Fourth Wing
Book #2 Iron Flame
Book #3 Onyx Storm