Iron Flame by Rebecca Yarros

࣪ ִֶָ☾. Iron Flame picks up right after Fourth Wing's explosive ending. Violet Sorrengail returns to Basgiath War College for her second year, now burdened with dangerous secrets about the venin and the truth behind Navarre's wards.
The new vice commandant, Major Varrish, makes it his mission to break Violet unless she betrays Xaden. Meanwhile, her relationship with Xaden is tested by distance, his refusal to share information, and the arrival of a figure from his past.
As survivors of Athebyne are hunted down and the venin threat grows, Violet digs into ancient history to find a way to protect everyone she loves. With dragons, political intrigue, and heart-pounding action, this sequel raises the stakes and ends on a jaw-dropping cliffhanger.