The Arcane Heart by Caroline Gibson

Prince Lonn swore he’d never buy a slave. That was before he met Llias.

Lonn carries a heavy burden; it is his destiny to inherit the powerful throne of Otharn and rule over the joined worlds. Suffocated by his overbearing parents and exhausted by the cold-blooded plots that surround him, Lonn escapes his troubles on a mercy-mission to the far-flung world of Tornatt. Fate has other plans for him, however: at a filthy slave-market Lonn finds a man his heart cannot leave behind, no matter what the price.
Llias of Fryst has no care for fate, destiny, or the desires of arrogant princes. Kidnapped from his home planet by a malevolent witch, Llias refuses to be a pawn in anyone’s game, even someone as handsome as Lonn. Llias has plans of his own, and none of them involve being Lonn’s grateful, obedient, companion.
Trapped together on Tornatt, Lonn and Llias are forced to wrestle with fate, destiny, and the manipulative plots that surround the Otharnian throne. When the witch finally shows her face, Lonn must make a choice; betray his family and risk the safety of the joined worlds, or lose Llias forever.