Bad Blood by Jenna Wolfhart

Duty. Desire. Destruction.
To Selene, duty is everything. She is the last of her kind, a royal descendent of the original generation of vampires. And as the final Titan queen to rule the kingdom of Troy, she is the only one standing between her people and the cruelty of Zeus.
When her enemies—the twelve Olympian vampire monarchs—summon her to participate in their annual blood ceremony on a remote island, she has no choice but to attend. It’s a requirement in the treaty.
But one by one, the Olympians start to show up dead. It seems a monarch has broken the pact of peace and is trying to take out all the others, including Selene. And when she tries to escape the island, she finds all the boats have been destroyed. She’s trapped.
Worse, dark desires stir in her heart for the one king she knows wants her dead. Ares, bloodthirsty for war, tried to kill her months before. And even now, his touch feels like the heat of battle—or something more.
Before she loses her life, her kingdom, and her heart, Selene must do whatever it takes to win this vicious game—even if it means destroying every single monarch on the island, including Ares.