When the Second Host of the Celestials sank the Deviant Lemuria in what is known as the "Great Cataclysm," Ikaris guides a ship of humans to safety. The humans mistake Ikaris for a bird, and (per Marvel Universe history) he is memorialized as the dove that guided Noah to the Mountains of Ararat (Eternals vol. 1, #2).
When his father disappears while fighting the Deviants, the young Icarus seeks him out using the mechanical wings. Too inexperienced to fly on his own, young Icarus soars too high, loses consciousness in the upper atmosphere, and falls to his death. Finding his son dead, the father Eternal takes the name of his son, Ikaris, in his memory.
Around 1000 AD, Virako, Ikaris' father, dies in battle against the Deviant Dromedan. Ikaris is then adopted by his uncle Valkin, who reveals to him his secret Arctic home, the Pyramid of the Winds.
At some point during the early centuries, Ikaris and the Eternals would come into conflict with the immortal mutant, Apocalypse. This conflict would end, and Ikaris and the Eternals defeat him, and Ikaris believes Apocalypse dead.
In 1823, as part of the Eternals' preparation for the coming of the Fourth Host of the Celestials, Zuras orders Ikaris to perfect his cosmic senses in anticipation of their coming. When they draw near, Ikaris is ordered to go to the City of the Space Gods and revive the sleeping Eternal Ajak to greet them.
When the Celestials arrive on Earth around a century and a half later, Ikaris succeeds in this mission with the aid of human archaeologist Dr. Daniel Damian and his daughter, Margo. Ikaris witnesses the arrival of the Fourth Celestial Host on Earth. He then publicly reveals himself as an Eternal. He later battles a "cosmic Hulk" robot, and fights Dromedan. During the Fourth Host, Valkin's son Druig captures and tortures Ikaris to force him to reveal the location of the Pyramid of the Winds. Ikaris catches up with Druig and kills him, but not before Druig is able to fire a weapon at a Celestial; the Celestial, however, is unharmed.
Later, he encountered Thor for the first time. He next participated in a battle between the Eternals and the Olympian gods defeating Ares. Not long after that, he learned of the demise of Zuras, the Prime Eternal (leader of Earth's Eternals).
Later, it was revealed how Ikaris first encountered the Inhumans. Ikaris was later captured with the other Eternals by a Deviant army. He joined with the other Eternals and Iron Man (Jim Rhodes) in defeating the Deviants. After that, Ikaris battled Maelstrom alongside the other Eternals and the Avengers. After the battle, he was selected by the Uni-Mind to remain on Earth.
During his long lifetime, Ikaris also attempts to be a fair and just leader of the Eternals, but has abandoned Eternal traditions to take an active and public role in protecting the human race. He has even adopted a unique human identity by becoming a professional wrestler with the Unlimited Class Wrestling Federation as "Iceberg" Ike Harris. Ikaris and fellow Eternal Thena have had a strong dislike for each other for a long time, and find themselves coming to conflict when she is appointed as Prime Eternal. Questioning her worth as Prime Eternal, Ikaris faces her in the Hall of Eternal Judgment and defeats her, becoming the new Prime Eternal. Ikaris then battled Ghaur alongside the Eternals, Thor, and the West Coast Avengers. He also buried Margo Damian when she was killed.
Later, with the other Eternals, Ikaris captured the Silver Surfer on behalf of the High Evolutionary. Some time after that, he was captured by the Deviant leader Brother Visara, and then freed by Kro. Ikaris exposed Dr. Damian as the Eternals' enemy, and battled a transformed Ajak as a result.
In The New Eternals series, Ikaris still retains the title of Prime Eternal. Ikaris and the Eternals' ancient nemesis Apocalypse returned to detonate a nuclear warhead on Deviant Lemuria. He causes Ikaris's father Virako to return to life. Ikaris confronts, and fights Apocalypse. Although Ikaris is defeated by Apocalypse, Ikaris still manages to destroy his ship and thwart Apocalypse's plan. Virako is appalled to find the Eternals accepting Deviants such as Ransak the Reject and Karkas.
Under the alias of "Sovereign", he introduces the Eternals to the world as a team of super-heroes called the "New Breed".