Antigoddess by Kendare Blake

The Goddess War begins in Antigoddess, the first installment of the new series by acclaimed author of Anna Dressed in Blood, Kendare Blake.
Old Gods never die...
Or so Athena thought. But then the feathers started sprouting beneath her skin, invading her lungs like a strange cancer, and Hermes showed up with a fever eating away his flesh. So much for living a quiet eternity in perpetual health.
Desperately seeking the cause of their slow, miserable deaths, Athena and Hermes travel the world, gathering allies and discovering enemies both new and old. Their search leads them to Cassandra—an ordinary girl who was once an extraordinary prophetess, protected and loved by a god.
These days, Cassandra doesn't involve herself in the business of gods—in fact, she doesn't even know they exist. But she could be the key in a war that is only just beginning.
Because Hera, the queen of the gods, has aligned herself with other of the ancient Olympians, who are killing off rivals in an attempt to prolong their own lives. But these anti-gods have become corrupted in their desperation to survive, horrific caricatures of their former glory. Athena will need every advantage she can get, because immortals don't just flicker out.
Every one of them dies in their own way. Some choke on feathers. Others become monsters. All of them rage against their last breath.
The Goddess War is about to begin.
At the Publisher's request, this title is being sold without Digital Rights Management Software (DRM) applied.