Black Gate

Katherine Green has spent the last few years of her life hiding what she really is from humans who would use her power for their own gain. As a member of the elite and secretive Casper Project, she spends her days tracking down aliens, alien artifacts and weaponry for the government and she's just found an alien artifact so terrifying, so deadly that there is only one man on Earth who can stop her from destroying everyone and everything--but he isn't exactly human.

Teagh has been the Guardian of the Gate for centuries. Known as the Dark One, he lives a solitary life hiding from power hungry humans and immortals both. He’s a guardian, not an assassin, but when he’s sent by an Immortal Seer to stop one human female from unleashing hell on Earth, he’s too late. Kate is already on the other side, and the Gate no longer obeys him. Its allegiance has switched to the dangerous Timewalker woman he doesn't dare trust, but aches to keep for himself.

Black Gate is a sci-fi action adventure with a romantic twist! The story is a stand-alone time travel adventure with kick-a$$, sarcastic heroine, a smoking hot, alien hero and a HEA. (*Previously released in 2014 under the same title)

Don't miss the rest of the 5 book series - the Timewalker Chronicles: 
Red Night (Book 1) 
Silver Storm(Book 2) 
Blue Abyss (Book 3) 
Black Gate (Book 4) 
White Fire (Book 5)