WARNING: The following contains spoilers for Eternals: Celestia #1, available now from Marvel.
The Eternals and the Avengers are both typically heroic forces that work to protect the Earth. Even if they operate on very different scales, they are nominally on the same side. But that might be about to change in a potentially very brutal fashion.
The Avengers might be on a crash course for a confrontation with the Eternals due to the revelations made in Eternals: Celestia #1 by Kieron Gillen, Kei Zama, John Livesay, Matthew Wilson and VC's Clayton Cowles -- and they don't even fully realize it.
Ahead of their Marvel Cinematic Universe debut, the Eternals have been given a fresh perspective on their function in the universe. Created by the Celestials to protect and monitor the planet Earth for eons, they've only recently begun to recognize the full scope of their choices and actions -- especially now that the Celestials have ceased informing them. Ajak, one of the chief priests of the Eternals, has been going through a crisis of faith over the issue. In the present day, she speaks with her fellow Eternal Makkari -- whom she has over the millennia come to see as a heretic to the Eternals and their connection to the Celestials. Notably, Ajak also thinks back to ancient times, thousands of years ago when she confronted the first Avengers the world ever knew.
Odin, Phoenix, Starbrand, Agamotto, Iron Fist, Ghost Rider and Black Panther came to the throne of the Celestials and found themselves confronted by Ajak. A brawl quickly broke out, with Ajak witnessing their power and allowing herself to die to gain knowledge about them and their potential. Restored afterward at the cost of a Deviant's life, Ajak spoke with the rest of the Eternals. Although she believes the Avengers of that era to be an inelegant group, she saw them as likewise defenders of Earth who could assist in their overall mission. It is only these words that stall the army of the Eteralns, who'd been prepared for battle. But in the present day, upon seeing the corpse of the Celestial Projeniter turned into Avengers Mountain, Ajak is consumed with rage.
Even after learning the Celestials allowed this to happen -- and communicated with the Avengers while ignoring their self-admitted priest -- Ajak decides she can not allow such indignities to stand. She even moves to attack Ghost Rider outside the Avengers Mountain but is held at bay by Makkari. Despite the attempts by Makkari to make her see otherwise, Ajak decides that her great mistake was allowing the original Avengers to live -- leading to the current inheritors of their power. This sets up the Eternals to potentially come to blows with the Avengers over their recent advancements and developments, giving the Earth's Mightest Heroes another enemy to contend with. The Avengers have already been distracted by global conflicts, earning them foes all over the world.
The Kingdom of Atlantis grows restless, and the Russian Winter Guard has captured and corrupted Hulk into their agent. The Vampire Nation has been making moves behind the scenes, while the United States government (and their now-defunct Squadron Supreme) likely still have tensions with the Avengers. Now, they might also have the Eternals going up against them. On top of likely dividing the loyalty of the Eternals -- with both Ikaris and Sersi having deep connections to the Avengers -- it presents another threat to the heroic team. A battle between them and the Eternals could end up engulfing the entire world, and an Avengers/Eternals war could even be more destructive than the conflict that arose between the Avengers and the X-Men. It's a potentially ruinous prospect -- and one Ajak might well embrace.