Ghost Town - Hound of Hades, #3

Mal chose compassion over duty. Now the city is paying the price. Two spirits are haunting the streets of Manhattan, and one of them has a body count. Victims have been turning up all over the city, each killing more vicious than the last, and the only way to stop it is to send the spirits back where they belong-before they grow too strong for even Hades to control. If Mal can't return them to the afterlife, she'll pay a price of her own.
They're only here because she disobeyed an order-and broke the laws of the underworld. For this betrayal of her god, only one punishment exists: she will be stripped of Hades's Mark. And the only way to remove a god's Mark is to destroy the bearer's soul.