Dark Unicorn. Horn Sworn, #1

Perfect for fans of Sarah J. Maas and Victoria Aveyard, Dark Unicorn is the first of the new and mysterious adult fantasy series Horn Sworn. Princess Wrenyve knows her oath to return her Kingdom's missing artifact, the Heart of Spellshallow, means one of the King's children will die. Even if Wren is cunning, fast, and strong enough, there's a good chance it will still be her. Without the Heart, magic will flee the land faster than the night flees the day.
Without the Heart, Spellshallow, and everything in it will cease to exist. As her brother's shameful thievery is uncovered, Wren must swallow her pride and accept help from allies whom she knows she can't trust: Thackery is the ambitious son of the richest and most powerful councilman--her very ex, scorned lover--and the wounds Wren caused him are still raw. Calen isn't a boy at all. Trapped in human form after a hideous defeat, Calen may have lost his horn, but is still a unicorn--a self-serving, treacherous scoundrel, as dangerous to any man who crosses him as he is to the innocence of every fair maiden.
Luckily for Wren, she was already doomed. To find the Heart and restore the magic, to retrieve the lost horn, Wren and her unfortunate travel companions must survive three separate realms--each more deadly than the last. Unknown enemies and strange magic hinder their way, and yet the most perilous thing is what lies within their own hearts.