Book Onyx Storm — Rebecca Yarros

After nearly eighteen months at Basgiath War College, Violet Sorrengail knows there's no more time for lessons and no more time for uncertainty - the battle has truly begun. With enemies closing in from all corners, they need an army. They need power. And they need to unravel the dark truth about what's really happening on the continent before it's too late.
Violet must embark on her most dangerous mission yet and travel beyond the failing wards to seek allies who will stand with Navarre. The trip will push her to the very edge of her power, and the odds of dying are far greater than the hope of success. But Violet will risk everything to save her dragons, her family, and him . . . even if it means keeping a secret so big it could destroy them all.