Vows in Violence by Vi Carter

The Commission has sent Azrael, their deadliest assassins, not knowing that a monster already prowls the streets of New York City.
Ivan Romanov.
After years of waiting patiently in the shadows, I’ve finally come to claim what is mine. And that prize is Viviana Valachi, the youngest daughter of the Valachi Don.
She is timid and obedient, everything her father raised her to be.
But there is a fire in those Valachi eyes that is matched only by her brother, whom I hold captive.
Azrael is on the move. No place or person is safe, even those I keep in golden cages.
We must pay for the sins of our fathers—this is the one lesson we have learned in the Five Families. Angel Valachi has thrown our world into chaos, and I have inherited the duty of restoring order in the only way I know how.
Sin pays for sin. Blood must pay for blood.
I didn’t expect my pet to work herself into my body, mind, and soul. As much as I want to feast on the spoils and carnage of this war, I can feel the leash she is wrapping around my throat.
As much as I want to fight it, I find myself captivated by her guidance.

The hunt is on, and none shall sleep for a monster is on the loose.