From the Mouths of Sirens by Abigail Hair

Win the Tournament. Kill the Prince. Take the Crown.

Since childhood, Princess Saoirse has trained for the sole purpose of being chosen for the Tournament, a gladiator-style competition in which four enemy nations compete for the right to rule. An angel-like people known as the Aura have won the Tournament for a century, enacting exploitative treaties and ruling the continent with a merciless grip. If Saoirse survives the three infamous trials, she will win the Crown of Revelore and overthrow the nation responsible for her mother’s death.
But when Saoirse is denied entry into the Tournament by the Mer king, she makes a bargain with an imprisoned sea witch. In exchange for entrance into the competition, Saoirse agrees to kill the Auran prince and bring back the witch’s stolen dagger. But while fighting for her life in the arena, the last thing Saoirse expects to do is fall for her greatest enemy, the arrogant Prince Rook.
When Rook and Saoirse uncover the sinister secrets of their ancestors, they realize the animosity between their people is rooted in long-forgotten lore. What once was a competition for the Crown evolves into a perilous ancient game in which Saoirse and Rook are both pawns. As twisted truths are revealed and myths become real, Saoirse must decide if she should kill the man she is falling for and avenge her people, or spare his life and face the wrath of the Titans.