Slaying the Vampire Conqueror (Crowns of Nyaxia) by Carissa Broadbent

She was commanded to kill him with a single strike to the heart. She didn't expect her own to betray her.

Sylina has sacrificed everything for her goddess--her soul, her freedom, her eyes. Life in service to the Arachessen, a cult of the Goddess of Fate, has turned Sylina from orphaned street rat to disciplined killer, determined to overthrow Glaea's tyrannical king.

But when a brutal vampire conqueror arrives on their shores, Sylina faces an even deadlier adversary. She's tasked with a crucial mission: infiltrate his army, earn his trust...and kill him.

Atrius is a terrifying warrior carving an unstoppable path through Glaea. Yet when Sylina becomes his seer, she glimpses a dark and shocking past--and a side of him that reminds her far too much of parts of herself she'd rather forget.

Sylina's orders are clear. The conqueror cannot live. But as the blood spilled by Glaea's tyrant king runs thicker, her connection with Atrius only grows stronger. A connection forbidden by her vows. A connection that could cost her everything.

Crowns of Nyaxia Reading Order:
The Serpent & the Wings of Night
The Ashes & the Star-Cursed King
The Songbird & the Heart of Stone

Crowns of Nyaxia Standalones
Six Scorched Roses (short novel)
Slaying the Vampire Conqueror