A vow to uphold (слэш)

On a night of tragic endings and new beginnings, two fathers make a vow. Sixteen years later, Yuuri learns the truth that everyone has been hiding from him - he's a prince, the child promised to the usurper's son to bring the Katsuki line back to its rightful place. Now it is time for him to make the choice: marry the alpha he doesn't know for the good of the kingdom that thinks him all but lost, or forget about it and live the rest of his life second-guessing his choice and wondering about what might have been.
Almost four years later, he makes the decision that will change his life forever.