Chain of Illusions (Bringer and the Bane) by Boone Brux

Rell has lived in the Shadow World for thirteen years as a Demon Bane, the formerly vivacious young woman now the demonic enemy of the immortal Bringers. But when she is given the chance to become human again―and a full-powered Bringer―Rell isn't sure if it's better to be a demon in the Shadow World or to risk her life for her humanity.

For two years, Siban had been tortured in the Bane prison, only to fall in love with the beautiful demon who helped him escape. Tormented by the thought of never seeing her again, he maintains a life of solitude. So, when Rell's human body is rescued from the Shadow World, he will do anything to be with her―even if it means challenging death to become a Bringer, too.

But once their Bringer transformations and training are completed, Siban and Rell must join a mission to go deep in the Shadow World to rescue others trapped by the Demon Bane King. And what they discover destroys everything they knew about the Demon Bane. The lovers must learn to trust themselves, each other, and their new powers if they're going to make it out alive.