Subaru's End Series by Victoria Clapton

After discovering that all she fights for is a lie, a young commander joins her arch nemesis to defeat her king and save the three realms.Cahya is a commander in the army of Refulgent. Her mission is to defend Hikari, the Light Realm and defeat the Soul Smite army of Kurai, the Dark Realm. Yet, Cahya has a secret, and her world unravels on a chance meeting with the King of Kurai where she discovers she may have helped fracture the realms.

Imprisoned in Hikari, Cayha’s last-ditch effort to reason with Leor, the King of Light, fails, but her efforts aren’t in vain. Hidden in plain sight in the rotunda of Halcyon Palace is a secret—a magical secret, and Cahya must unlock the mystery so that she can return to Kage and the dark realm of Kurai and together, with Tariq and their allies, mend what has broken the celestial realm of Subaru.