War of Her Heart by Lexi Hodges

Violet has lived her entire life in the Mountain Realm and has been in a secret relationship with Calum, the Sovereign of her realm, which is completely forbidden. The rule is clear: Every Sovereign must marry from the royal bloodlines of the Seven Sovereigns.
When Calum is sent to the Night Realm to begin his engagement to the sister of the Night Realm’s Sovereign, he promises Violet he will find a way out of the engagement and make his way back to her. Violet holds onto that promise until she finds herself in the Night Realm where she realizes everything isn’t as true as she thought it was.
To make matters worse, the Sovereign of the Night Realm—the most evil and vicious of them all—has taken a strange liking to Violet and is quite curious about her rare heart condition which is something unheard of amongst fae.

Now Violet must navigate her way through the next several months as she fights an internal battle between her heart and her head.