Crown So Cruel by Emily Blackwood

The Scarlata Empire has risen, but for the angel of the blood kingdom, peace never lasts long.

Jessiah has spent the last two years commanding a new army of vampyres, so when a neighboring kingdom threatens their home, he is ready to strike.
But his king and queen hesitate to dive into a new war so soon. They’ll send Jessiah to investigate the threat under one condition—he brings Rummy with him.
Rummy is the blood queen’s best friend and personal advisor, but she also happens to be the one person Jessiah can’t stand.
The two are forced into an unexpected alliance as they travel across the kingdom and encounter challenges they never saw coming. Will they put their mutual abhorrence aside for the sake of Scarlata? Or will this new threat expose the truth behind their masks, demolishing the home they’ve built along with it?