Kingdoms of Life & Death by Veronica Lancet

A reckless princess.
A mysterious demon.
An unlikely master-slave bond.

Kissa Andreos had two goals.
1. Become the best mage Krateos had ever seen.
2. Capture the affection of her brooding demon slave, Caean.
Too bad that she wasn’t strong enough for the former, while the latter was considered a capital offense punishable by death.
But she wasn’t deterred.
In a world where demons were used and discarded for their powers, Kissa saw Caean as more than a tool. He was her best friend and the sole object of her affection.
Unfortunately, he didn’t return her feelings.
When a conspiracy against Kissa comes to light and new dangers abound, Kissa and Caean must become closer than ever in order to defeat an invisible enemy.
Can Kissa outsmart her enemies while nursing her unrequited love from a distance? Or can she finally convince Caean to take a chance on her—even if the entire world would shun them for it?

Kingdoms of Life & Death is the prologue to the Catharsis Trilogy, which follows Kissa and Caean. This is an extreme slow burn high fantasy that ends on a cliffhanger.