A Mask of Silver Fire by Evelyn Arden

Each person has their own runes that hold all future events. Trading runes changes the future. The sun god emperor is the most powerful rune holder in the world.

Released from years of imprisonment, Vala knows only one thing. To survive in the Imperial Capitol and stay unnoticed by everyone. By day, she’s a janitor. At night, she secretly designs rune trades for the government to support her friends.
When a masked man tries to kidnap her, she is forced to confront the truth. That her runes will never be unwanted and that very powerful forces want her, alive or dead.
She tries to flee the capitol but is arrested by the emperor’s soldiers. The emperor demands she work for him, but his request soon turns seductive, and she begins to realize just how deeply her future is entwined with an upcoming eclipse threatening to destroy the emperor’s runes and topple his power.
Navigating a ticking clock and palace politics, she must use her brains to save the emperor while facing increasing condemnation by the masked man who haunts her footsteps, his past as mysterious as his continued warnings.

Yet betrayal awaits her from every direction. She must face a terrible decision, one that will cost her everything she holds dear, including life itself.