Doctor Who also has a race of beings called the Eternals, who also have nothing in common with the Marvel characters apart from being immortal and superpowered…
You’re infringing the trademark if a typical member of Marvel’s target audience might be confused as to whether your book is about the Marvel characters, or is an official Marvel product. My understanding is that confusion is more likely if you mention “Eternal” or “Eternals” in the title or the blurb than if you just mention them in the text of the book.
Marvel are owned by Disney nowadays, who are very protective of their rights. Trademark law, at least in the USA, has a “use it or lose it” rule - if a trademark owner sees someone infringing on their mark, they have to take action to stop the infringement. If they don’t, future infringers can point to this lack of action as evidence that the owner doesn’t care about protecting the mark. If a court agrees with that assessment, they can decide to invalidate the mark.
So I guess this is a “do you feel lucky, punk?” situation - are you sufficiently attached to the name to want to put yourself in the sights of a company with deep pockets and a legal obligation to make you stop if they notice what you’re doing?