The Immortals by Jordanna Max Brodsky
Summary from GoodReads:
MANHATTAN HAS MANY SECRETS.
SOME ARE OLDER THAN THE CITY ITSELF.
The city sleeps. Selene DiSilva walks her dog along the banks of the Hudson. She is alone-just the way she likes it. She doesn’t believe in friends, and she doesn’t speak to her family. Most of them are simply too dangerous.
In the predawn calm, Selene finds the body of a young woman washed ashore, gruesomely mutilated and wreathed in laurel. Her ancient rage returns. And so does the memory of a promise she made long ago. To protect the innocent-and to punish those who stand in her way.
With the NYPD out of its depth, Selene vows to hunt the killer on her own. But when classics professor Theo Schultz decodes the ancient myth behind the crime, the solitary Huntress finds herself working with a man who’s her opposite in every way. Together, they face a long-forgotten cult that lies behind a string of murders, and they’ll need help from the one source Selene distrusts most of all: the city’s other Immortals.