The Shadow Prince by Helen Scheuerer

The world has changed. There are new gods roaming the earth, and Lucian and Artair, two souls forced to share one body, are prime targets for their wrath.

Between them both is Elver: a monster girl caught between the humanity that Artair brought back into her life, and the strange obsession that Lucian stirs within her. But she is no longer the girl that she was when they first met. Elver has been changed, too - in ways more terrifying than even she can imagine.

There is treachery at work in their midst, and as the world teeters on the edge of chaos and devastation, the battle lines must be drawn before it's too late.