The Dark King's Heart by Lyssa Lund (Shadows of Dark and Light 1)

Enter a world of magic and prophecy, where twin sisters are destined to shape the fate of two kingdoms. Twin sisters, one prophesied as light and one as dark, are thrown into a dangerous game of power and love.

Niomi, the dark sister, is kidnapped by the Dark King of the Unseelie court, who intends to make her his wife to save his kingdom. Despite his initial cruelty, Niomi discovers that Callan's heart is good and that he is fighting to save his people. As they work together to stop a war between the Seelie and Unseelie courts, Niomi and Callan grow to care for each other, proving that appearances can be deceiving.

With the fate of two kingdoms in their hands, the forbidden love between the Dark King and the dark sister will be tested in this epic tale of magic, power, and love. Niomi and Callan must help both sides see the truth to save the ones they love before it's too late.