The Demon's Kiss - Bloodcaster Chronicles, #1

An assassin with a secret. An angel with a strange power. A war that threatens to destroy them both. Cora is an assassin who never misses her mark. But she has a deadly secret: her blood can create the most powerful magical elixirs in the world. And she's been hunted for it her whole life. When a powerful demon blackmails her, threatening to expose her secret, she has to do one last job to buy her freedom: kill a Nephilim named Vince.
Cora figures this will be an easy target. Until Vince time travels, appearing in two places at once. Thwarted, Cora must get close to Vince so she can learn the secret to his strange ability--and finally kill him once and for all. But the closer she gets to Vince, the more her icy heart begins to thaw, making way for new emotions she's never felt before. As she falls for him, her secret is exposed, igniting a powerful war between demons and Nephilim.
In the end, Cora must choose between her own survival and preventing a war that will tear the magical world apart.