King of Avalon by Vivienne Savage

Fated to love. Fated to die as enemies. The fae told her killing him was the only way to save the world.
They lied.

From one age to the next, the immortal King Arthur worked alongside his knighthood to protect the world from evil dragons—until a fellow knight betrayed him and a dark witch trapped his soul.
Long ago, Camelot knew the Nimue as the Lady of the Lake. In modern day Manhattan, she’s the proprietress of Manhattan’s hottest paranormal lounge. For a fae, life couldn’t be more mundane, until a half-naked, reincarnated King Arthur stumbles out of the future and into her life again to prophesize the world’s end.

They were once lovers. Now the faerie courts will want him dead for perverting destiny, and they’ll want her to carry out its will.