Thena is a second generation Eternal (evolutionary offshoot of humanity created by experiments performed by the Celestials), being the eldest daughter of Zuras, leader of the Eternals of Earth, and Cybele. Although she has lived for over 4,000 years, Thena is still considered a young woman by Eternal standards. At her birth she was named Azura in honor of her father, Zuras.
When the civilization of ancient Greece began to rise, the Olympian god Zeus decided to make the presence of the Olympian gods known to the Greeks so that they would worship him and the other gods. The principal nexus between the dimension of Olympus and Greece lay atop Greece's Mount Olympus, near Olympia, the capital city of the Eternals. Zeus and his daughter Athena (Roman name Minerva) held a meeting with Zuras and Azura, who had become her father's principal advisor. Noticing the marked physical resemblance between Zeus and Zuras and between Azura and herself, Athena suggested that the Olympian gods and the Eternals form an alliance in which the Eternals would act as the gods representatives on Earth. Zeus, Zuras, and Azura agreed, and Zeus gave Azura her current name of Thena to signify the sealing of the pact, and also to indicate that she would serve as Athena's personal representative. However, over the many years, many humans came to think of several Eternals not as representatives of the Olympian gods, but as the gods themselves. Thena was constantly mistaken for the goddess Athena, for example. All of this confusion led to a growing resentment by the gods towards the Eternals, which eventually erupted into a war, during which there was a bitter combat between Thena and Athena. However, the war ended the very same day that it began, and the Olympian gods and Eternals are now at peace.
In ancient times, Thena first encountered Kro, warlord of the Deviants, another offshoot of humanity created by the Celestials. At some point, Thena and Kro became strongly attracted to each other, but their two peoples were often in conflict. Thena even accompanied Kro to the Deviants city on the undersea continent of Lemuria, but the tour ended badly, along with their relationship. Until recent years Thena had not seen Kro for a millennium, and had believed him to be dead. However, Kro is unusually long-lived for a Deviant, a secret he never revealed to his own people.
During World War II, Kro served Deviant interests by funding various armies and criminal enterprises. Thena advised Zuras to send the Eternal Makkari to deal with Kro, and Makkari did so under the alias of Hurricane. At one point, the Eternals encountered the Inhumans, a race whose powers had come from experiments by the extraterrestrial Kree, and Thena joined the Eternals in helping the Inhumans relocate their city of Attilan from the North Atlantic Ocean to the Himalayan Mountains.
During one encounter between Thena and Kro, the two made love and Thena became pregnant with twin children. To conceal the affair from her people, Thena kidnapped an infertile human woman and placed the twins within her womb so that they could be raised as humans. When the Celestials sent their Fourth Host to Earth in order to judge the worthiness of humanity, the Eternals and the Deviants, Kro led a Deviant force in attacking humanity in hopes of provoking them to assault and principal advisor, assumed his position as leader of the Eternals of Earth. the Celestials. Thena confronted Kro and helped bring an end to hostilities. Kro extended another invitation for Thena to visit Lemuria, and she accepted. In Lemuria, Kro introduced her to Ransak the Reject, a deviant who was virtually human in appearance, giving Kro hope that his people's curse of genetic instability could be overcome. Although Ransak was wild and distrustful, Thena won his confidence. She also met the Deviant Karkas, who proved to have the gentle soul of a poet that belied his monstrous visage. Thena brought Ransak and Karkas to Olympia to live among the Eternals, and she helped tutor Ransak to become more human.
After the arrival of the Fourth Host of the Celestials, the Eternals and Deviants decided to reveal their existence to humanity. At this time Thena publicly revealed herself to be an Eternal. However, the general public tends to disbelieve the existence of these "hidden races". Thena encountered Kro again when she, Karkas and Ransak joined fellow Eternals Ikaris and Sersi in exploring a Deviant outpost beneath New York City, and were aided against the Deviants by the Asgardian god Thor. Thor returned to Olympia with the Eternals and fought by their side when his own father Odin sided with the Olympian gods in assaulting Olympia, believing the Eternals to be pawns of the Celestials. Thena briefly sparred with Athena, but the battle was finally called off. The Eternals subsequently merged into the Uni-Mind to help Odin and the Asgardians confront the Celestials, but when the Celestials destroyed the Uni-Mind, the psychic backlash killed Zuras. Zuras' body was returned to Olympia, and eventually his power passed from him to Thena. His last message to Thena was that the Eternals' destiny lay off-world, and most of Earth's Eternals departed in the form of a Uni-Mind to explore the cosmos.
Thena claimed the full power and title of Prime Eternal in her father's stead, but fell under the sway of a mind-altering brain-mine that Kro had planted on her. Kro became the Deviants ruler, but soon found that the true power in Lemuria lay with the priesthood led by Ghaur. As Ikaris led some of the Eternals against the Deviants in violation of Thena's orders, Thena herself helped Kro escape Ikaris, finally revealed her deception before the Eternals, and helped Kro escape the priesthood. Thena and Kro found themselves pursued by both of their peoples, and Ikaris bested Thena in combat for the title of Prime Eternal. When Ghaur attempted to revive the Dreaming Celestial, a rogue member of the Celestials, Kro removed the brain-mine so that Thena could regain her full power and help the Eternals prevent the Celestial's awakening.
Kro eventually gave up the throne of Lemuria and a new regime emerged. The new rulers were tricked by a false scroll into believing that the offspring of an Eternal and a Deviant would destroy them, forcing Thena to confront the twin children she had given up before their births. Thena's now-teenage children, having been raised as Donald and Deborah Ritter, learned their true heritage from Thena and were introduced to Kro as well. The scrolls identifying the children as threats proved to be forgeries created by Dr. Daniel Damian, a one-time ally of the Eternals who blamed Ikaris for his daughter's death.
Thena and Kro both kept a watchful eye on their children from then on, and the children were endangered again when Maelstrom's minions attempted to use their lives to revive Maelstrom, but the Fantastic Four helped the Eternals rescue the twins. The twins were also imperiled by Ghaur when he attempted to use them alongside captured Eternals in an "Anti-Mind" to challenge the Celestials, but Kro and the Heroes for Hire helped save them. Thena took permanent custody of the twins after that and searched for a sanctuary where they would be safe, ultimately deciding to stay with the Heroes for Hire. Thena became a regular member of the team, battling threats such as the Master and Exodus, but the group ultimately disbanded after they were purchased by the Stark-Fujikawa Corporation.
A sudden change in the Earth's atmosphere resulted in the resurrection of several Eternals whose body's molecules were reconstituted in Olympia, among them Zuras. When the Eternal Sprite sought to become mortal so that he could enjoy an ordinary life, he manipulated the Dreaming Celestial to refashion all of Earth's Eternals into humans with manufactured memories, ordinary lives and no recollection of who or what they truly were. Thena became Dr. Thena Eliot, wife of Thomas Eliot, and became an inventor employed by Anthony Stark. While and Thomas were attending a party at the Vorozheikan Embassy, Thomas was killed by a band of terrorists who took Thena captive. Eventually rescued by Iron Man, Thena began to recollect who she was. She finally regained access to her powers and most of her memories, and she joined other awakened Eternals in confronting the Dreaming Celestial, which was now in danger of reviving.
Through communication via Makkari, the Eternals learned that the Dreaming Celestial would remain in San Francisco to begin a new judgment on Earth, and that its awakening would eventually result in an invasion by the extraterrestrial Horde. The Eternals retired to Olympia to resume their guardianship over the Earth. Thena brought Joey with her to Olympia, in defiance of her father's wishes.