The Forest Kings Daughter

As a child, fey princess Klara snuck out of her father's castle and did the forbidden: she befriended a demon boy. On their third meeting, her new friend Zeru gave her a demonstone ring. He thought it was one of many in his mother's jewelry box, not to be missed. But the trinket was actually a dangerous and powerful artifact, which the demon queen will stop at nothing to get back.
Now, with the demons mounting attacks to retrieve the ring, the conflict between demons and faeries has intensified. After a decade of skirmishes, the demon army draws ever closer to the faerie stronghold. Klara wants to prove to her father that she is worth more than the demonstone she wields as a weapon. Zeru, her childhood friend, now enemy, has been waiting for the right opportunity to get the ring back and right the wrong he did to his people.