Thena and Kro, despite being members of two offshoots of humanity who have been traditional enemies, have had a relationship for over 25,000 years. During the Vietnam war, the two had a sexual encounter, and Thena discovered she was pregnant. Using her powers, she secretly implanted the embryos into an infertile human woman, and the two grew up unaware of their ancestry. When Dr. Daniel Damian, a human who was aware of the existence of Deviants and Eternals, sent a monster (formerly Ajak) to slay the two children, killing numerous other sets of twins in the process, Thena took the two to Olympia, the city of Eternals, but did not inform them of their heritage. However, even Olympia proved unsafe, and the monster captured the two and fled to Peru, where they confronted Damian. The Twins learned of their true origin, and were rescued by Kro and Thena.