Alien Warlord’s Slave by Aya Morningstar

Sturmir, the strongest of the Glacian warlords, has launched a sneak attack on the twin planets of Kroxa and Bastion. He’s ready for a long and brutal war, but then he finds something that ends the war in a day.
A pure human woman.
No one has seen a pure human in centuries, and the Empress of the twin planets is decided by purity of blood.
Sturmir makes sure she understands who is really in charge. He makes her swear to obey his every wish and whim, and then he makes her Empress.
Rebecca wakes up in the future, and on her first day in this strange new reality, a brutal alien warlord hijacks her shuttle. He ignores every other woman on the shuttle and singles out her. He presses her against the wall, his powerful hand gripping her neck, and he makes her swear that she will obey him.
She’s now Empress of the two worlds, but with Rebecca sworn to obey him—and frustratingly attracted to him—who is really in charge?