One of the biggest fan theories about Avengers: Endgame suggests Kronos, an Eternal, will make an appearance. This theory has stemmed from multiple sources, including the announcement in April 2019 by Marvel Studios President Kevin Feige that an Eternals film was in development and the presence of Thanos, himself an Eternal, in both Avengers: Infinity War and Avengers: Endgame.
With all the scuttlebutt about what's going to happen with the Eternals announcement, many are wondering who Kronos and the Eternals are. The Eternals have a rich comic book history, much of which is likely to spill over into the MCU. Just who is Kronos the Eternal, and will Kronos be in Avengers: Endgame? How does he tie into the strongest Marvel characters, and what might he become should he show up on screen?
Kronos's origin story first appeared in the pages of What If...?, which revealed he was an Eternal, an immortal race of beings. He became the leader of the Eternals by leading a rebellion against his brother, Uranos. While he was in charge, he fathered two sons, Zuras and A'lars.
The people of Earth have mistaken Zuras as the Greek God Zeus given their similar powers. He was the first Eternal to form a Uni-Mind, and was chosen to be the leader of the Eternals of Earth.
A'lars, who is also known as Mentor, is Kronos and Daina's second-born son. Unlike his brother, a warrior, Mentor became a scientist like his father. He settled on Saturn's moon of Titan and had a family of his own with the last surviving Eternal who remained there, Sui-San. Together, they had two children, Thanos and Eros.
In his very first appearance in the pages of Iron Man #55, Kronos saves the soul of a man named Arthur Douglas who's taken out by a barrage from Thanos's ship. Since he has a soul on hand and wants to rid the universe of his grandson, Kronos uses Douglas's essence to create Drax the Destroyer, whose comic book origin differs greatly from his cinematic one.
Drax was explicitly created to slay Thanos. At first, he is woefully incapable of carrying out his purpose, but he eventually powers up enough to take on the Mad Titan, and it's all thanks to Kronos's meddling.
Kronos was one of the original Eternals whose immortality was something he searched for to benefit the rest of his race. While experimenting with that very subject, an accident occurred, which had a major impact on the future of the Eternals and the Marvel Universe. During the investigation, a force field cylinder that contained cosmic energy detonated, causing Kronos to disintegrate completely.
When this happened, his longevity was spread to all of the Eternals who were nearby, imparting immortality on the rest of the Eternal race. The second effect of this atomization wasn't the destruction of Kronos entirely, but a transference of sorts into an astral form. He continued to exist in an ethereal manner and gained numerous amplified abilities and powers as a result.
When Kronos's body was vaporized in the mishap that turned him into a cosmic ethereal being, his powers got an upgrade. He became an embodiment of time itself and is now able to exert his will upon the flow of time.
Kronos only does this when it's required, however, so the full extent of his abilities concerning time distortion remain unknown.