Thanos defines the Eternals history, as the debate surrounding his eventual birth and his subsequent terror are key events in their history.
Warning! Spoilers for Eternals: Thanos Rises #1 by Marvel Comics below
The Eternals don’t make sense in their current form without Thanos. Marvel Comics recently revealed the Eternals’ history was defined by the birth of the Mad Titan, whose conception went against everything the ancient cosmic race previously believed. Without Thanos being born, the Eternals’ existence would significantly change as his arrival altered their course for eons to come. And they only have themselves to blame.
Marvel Comics has told the story of Thanos’ birth before, as his mother Sui-San tried to kill after seeing his horrid appearance for the first time. The Eternal was born with the Deviant Gene, altering his appearance to make him look less human-like, and more like an actual monster. Unfortunately, Thanos would never recover from the ridicule and shame he suffered during his childhood, eventually betraying the Eternals and killing his mother and later, his father Mentor. In the newest run of Eternals, Thanos’ backstory leading up to his birth is explored like never before, showing the Eternals messed up big time by allowing him to be created.