Once upon a time, among the bloodred trees of Thirstwood, a young forest princess became friends with a lonely boy from underground. He gifted her an amber ring, a worthless trinket—or so he thought—because no sooner did he slide it onto her finger than the queen of the underground and the forest king declared war.

Years later, Cassia is a crucial force in her father’s army, wielding her ring of light that can blind and disorient hundreds of enemies at a time. Then battle-hardened Zeru abducts her, planning to steal the ring back to fix his costly childhood mistake. Exhausted, terrified, and more than a little mistrusting, Cassia is forced to travel with Zeru to a place they both believed only existed in storybooks, one where their childhood friendship slowly rekindles into something much more. But it's only a matter of time before the war they’ve escaped comes for them, and a hidden threat to forest and underground folk alike grows in the shadows.