The Hopes of a Crystalline Heart by Kari Jeong

Thousands of years ago, a war between Light Elves and Dark Elves exiled the Dark Elves from their home and forced them into the underworld, the Evergloom. Now sworn enemies, they refuse to treat with one another, and if found on enemy land, they are either killed, imprisoned, or turned into a slave.

Present day, Aelrie Everstar is a Light Elf and personal guard to the high priestess. When a Dark Elf assassin kills her charge in front of her, she pursues him, intent on killing him and having her revenge. All does not go to plan, and she is thrust into the Evergloom with him, a dark and treacherous place where they must stick together to survive. However, they will need to learn how to trust each other first, if they can avoid killing one another. See how desire and duty collide, and how an act of kindness can bring hope to even the darkest of places.