The Half King by Melissa Landers

A king by day. Shadow by night…
The Great Betrayal changed everything for the Allied Realm. Long ago, the kingdom’s noble houses rose up against the goddess… and for their treachery, the firstborn of each noble family was cursed.

One with perilous beauty.
One with destructive knowledge.
One with insatiable bloodlust.
But the royal house Mortara received the worst affliction of all. For while the king exists during the day, he fades into nothingness at night…until his twenty-first birthday, when he will be lost to the shadows forever.

Now an acolyte has arrived at court. Like all the second-born children of the Allied Realm, she’s destined to serve the goddess and become a Seer…only Cerise Solon has no gift of foretelling. In fact, she has no magical gift at all.
Instead, she’s surrounded by courtiers and priests—smiling sycophants whose hearts are filled with secrets and lies. And at the center of it all sits His Majesty Kian Hannibal Mortara, with his haunting eyes, sharp tongue, and an unerring ability to send her pulse skittering at the worst possible moments.

Falling for him is unthinkable. Because the king is the last of his line, and as the specter of his twenty-first birthday—and the full force of his curse—approaches, the kingdom holds its breath.
But there’s only one way to save a dying king… and it lies with the one person who’s hiding the biggest secret of all. Cerise.