Just like the Zacian
has an Alternate Form. However, unlike the Cover Legends
of both Pokemon Sword & Shield
, Eternatus can only change forms during the Max Raid Battle
your trainer must have against it near the end of the Main Story. This Alternate Form of Eternatus is commonly referred to as its Eternamax Form, and it sees Eternatus becoming the strongest Pokemon (by far) in all of the World of Pokemon. Even the likes of Mega Rayquaza doesn't compare to that of Eternamaxed Eternatus.
Unfortunately, however, players can't use Eternatus
in its Eternamax Form after capturing it. There is no way to make it change its form, and this was an intentional move by the Pokemon Company due to Eternamax Eternatus being as strong as it is. When in its Eternamax Form, Eternatus is almost twice as strong as most Legendary Pokemon. It simply wouldn't make sense to let a Pokemon that strong be permitted to regular use in battle since it would mop the floor against any other Pokemon in a 1 vs 1 Format. That said, however, there is still the possibility (though quite remote) that players will one day have the chance to harness the ultimate power of Eternamax Eternatus.