Stone Heart by Katee Robert

In the city of Olympus, people only speak about Medusa in whispers. She’s Athena’s knife hand, the one sent when Athena wants someone to disappear. No one asks Medusa what she wants, but she owes Athena her life, and if staining her hands with blood is the only way to repay the debt, it’s a small price to pay.
Until Athena sends her after Calypso, the mistress of the rich politician Odysseus. As far as Medusa can tell, Calypso hasn’t done anything remotely worthy of a death sentence, and her conflicted feelings only get compounded when she finally sees the woman. Calypso is beautiful and cunning and she’ll do anything to keep her life—including seducing her would-be assassin.
What starts as a ploy to escape quickly spirals into genuine interest. Medusa is hardly the cold killer that rumors suggest, and Calypso is far more complicated than her reputation as a ruthless gold-digger.
But it doesn’t matter that they’ve finally found something special together. Athena will have her blood, and this time not even crossing the River Styx will save them…