Dark Time by Dakota Banks
Book 1, Mortal Path Series
Genre: Urban Fantasy
Book Burb from Amazon.com:
Three hundred years ago, she sold her soul to a demon. Now she wants it back.
For centuries, the woman calling herself Maliha Crayne has lived a second life—as an assassin for the malevolent creature who owns her soul. A haunted killer with the blood of countless victims on her hands, she has finally discovered a way to nullify the demonic pact that chains her: If she saves a life for every one she has taken, she will be free.
But if she fails, her punishments will be unspeakable, unendurable . . . and neverending.