The last Guardian of Humanity

800,000 years ago Earth was being defeated by the people of the underworld in battle.
Some of the humans were taken as slaves and were mated with which led to the discovery of how pure their genes were and how it makes their eternal posterity unaging. So every millennium as an exchange for Earth's freedom, the male of mating age was free to pick a woman from earth, who was free to come back once they are done birthing a child.
Nobody knew what the underworld people look like because none of the women who left ever came back.
Kimberly Clarkson had heard the little myth that made her curious 6 years old eyes widened, the thought of another world fascinating her but now, whenever she remembered the little tale no one could prove exists she Scoffs, knowing better. The underworld story was well, just a story.
She just turned 23, had a childhood sweetheart that adores her, and was finally having a chance to contribute her quota to society now she was working at her dream firm as a lawyer.
But what Kimberly doesn't know is that soon everything she has ever believed in is going to change because it is time for the underworld people to cash out on their agreement and Kimberly's name falls among the lots.