Warning! Spoilers for Eternals #12 by Marvel Comics
The Eternals just gave the Avengers a reason not to trust them, gifting the superteam the perfect excuse to hate the ancient race of heroes. In Eternals #12, Sersi reveals to the Avengers that Thanos not only returned but had become the Prime Eternal and was defeated without them knowing. When Iron Man questions how they couldn't tell them Thanos returned to their planet, Sersi says the Eternals are landlords to humans and admits they shouldn't be trusted.
The Avengers and Eternals have always had somewhat of a rocky relationship. After all, when Earth's Mightiest Heroes found out there was an ancient group of heroes who could have intervened and helped them in some of their greatest battles but chose not to, it understandably grew a wedge between them. On the flip side, the Avengers, including Iron Man, hated that the Eternals would act in cosmic fights the team had no control over or knew little about. While the Avengers and Eternals have fought side-by-side before, often, the Eternals act in their own interests and not on behalf of humanity. The most glaring example came from the MCU when the Eternals didn't even intervene when Thanos arrived on Earth, as it wasn't part of their mission.
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In Eternals #12 by Kieron Gillen, Esad Ribic, Matthew Wilson, and VC's Clayton Cowles, Sersi finally reveals the truth about Thanos' return, the Mad Titan becoming the Prime Eternal, and the Eternals themselves defeating the supervillain. Iron Man is shocked that the Eternals would tell them about Thanos, telling Sersi, «it's our planet.» Those words hit a nerve for the Eternal. Sersi tells him how can she
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