The Empty Throne by Megan Derr

The Kingdom of Cremisio is in turmoil—the royal family dead by assassination, a secret heir no one can find, and three enemy kingdoms fighting bitterly to each take control of Cremisio for themselves. If the missing heir cannot be identified and located, Cremisio will fall once and for all, and thousands of people will die with it.
Only one person knows the identity of the missing heir: Lord Cohea Szelis, the Duke of Lindquist, also known as the Fox of Cremisio. Notoriously honorable, impossible to break, leaving those who have captured him frustrated and helpless. Until one of his captors notices the ring he wears, a ring that marks him Bound, an old, illegal spell that binds two people together, body and soul. If one dies, so does the other. If they can locate the person Cohea loves enough to Bind himself to, they might finally get the Fox to break...
The task of finding that Bonded falls to Jethue Bittersea, notorious exiled traitor to the throne, and an assassin who has never failed. It will also give him a chance to do some hunting of his own for this mysterious heir who is the key to everything...