While it is not known to evolve into or from any other Pokémon, the player battles a powered-up form of Eternatus known as Eternamax Eternatus in the climax of Pokémon Sword and Shield. Prior to Generation IX, this form could also briefly be seen when it uses its signature move, Eternabeam, but the form itself could not be acquired legitimately in-game.
Therefore, no, Eternamax Eternatus cannot be obtained by the player without hacking; it was simply used for the climax of SwSh.
Source
As Bulbapedia says, however, you can see Eternatus in its Eternamax form temporarily when it uses its signature move Eternabeam in Pokémon Sword/Shield.
