The Stormbound. Calder March, #1

They told her the trials would kill her in a week. They were planning on it. Eluned Carrow grew up tracing maps of a borderland the Hollow had already half-swallowed, a place where the edges of the world quietly stopped existing. She is too small for the rider trials, too sharp-tongued for the highborn cadets, and far too poor to matter to anyone at the Vael Spire. So when a scarred, lightning-veined runt drake named Vaehl looks past every gleaming candidate in the Rookery and chooses her, the whole college goes silent.
Because Vaehl burns the wrong color. Faint, impossible white. A hollow-light aspect no drake has carried in a hundred years. It should be the making of her. Instead it makes her useful in the worst way. The Stormwall, the living storm holding back the unmaking force that ate her hometown, is failing. And the crown has been quietly patching it the only way it knows how: by sacrificing bonded rider-drake pairs and feeding them to the wall.
Lune's rare aspect makes her either the cleanest sacrifice in a century, or the one bond that could mend the wall without anyone dying at all. Castian Roen knows which. The cold, controlled wing-second who runs the trials, who seems to want Lune dead from the moment she arrives, is the only person at the Spire who understands what her bond truly is, and what the crown intends to do with it. He has orders.
He has a war he has already lost too much to. And he has a growing, dangerous certainty that the undersized mapmaker he was supposed to bury is the only thing that can save them all. Enemies to lovers. A bond shared unto death. A girl who was meant to be fuel, becoming the fire instead.