Soul-Strung by Rachel Hobbs

Battleworn Ruby is putting her darkest days behind her, and where better to start again than the bustling Callien city? Too good to be true, when the demon of her past comes crashing back into her life, the warning he carries with him leaves it in pieces. Where Drayvex goes, chaos and mayhem trail in his wake. And she’s bound to him tighter than ever before.
Scheming Drayvex has been busy: chasing the demon turned traitor from world to world, keeping furtive tabs on the girl with a piece of his soul; oh, and running a demon empire. Concerned Ruby will be discovered alive, he enlists her on a hunt for the lapis temporis, an object that can quite literally turn back time, to fix old mistakes and kill their mutual enemy in the past. He’s done playing games.
Power-starved Saydor is on the run. Converting to his cause one underdog at a time, his eyes and ears are everywhere. You can’t kill what you can’t find, and Saydor’s covert talent is deadly. The bigger you are, the harder you fall. And he’s just getting started.
But when you’re playing for time itself, how do you know when you’ve been here before?