Doesn't anyone feel like the plot of Sword and Shield feels like Final Fantasy 7's in Rose (hah!) colored glasses? With Macro Cosmos as the power company that runs everything, draining energy from the land itself. Well, that would make Eternatus the Jenova equivalent, being an alien that arrived on the planet millennia ago that the power company is obsessed with. And Eternatus sort of lives up to the hype.
Even disregarding the purposely overpowered Eternamax Form that should never see competitive play (you need that kind of defense to stand up to a 4v1 including Zacian Crowned), base 145 Special Attack and 130 Speed is incredibly potent. Most formats that allow legends tend to be a bit on the slower side, as many of the best legends top out around 90-95 Speed, so 130 Speed is amazing. Base 145 Special Attack and a moveset just wide enough to abuse it can turn Eternatus into an amazing sweeper/revenge killer. But its really Eternatus' other traits that push it over the top. Base 140 HP is absolutely titanic, with passable 95 defenses, Poison/Dragon typing that prevents Eternatus from being stuffed by Xerneas and other Fairies and access to Recover and Cosmic power makes Eternatus more bulky than Giratina Alter, with great offense even uninvested. Poison is unfortunate given the Psychic laden legendaries running around, but Eternatus is a huge threat that must be planned for lest it stuff your entire team.
Alien Miasma
Eternatus
Ability: Pressure
Item: Leftovers
-Toxic
-Recover
-Venoshock/Dynamax Cannon
-Mystic Fire
Nature: Bold (+DEF, -ATK)
EVs: 252 HP / 252 DEF / 4 SPE
Eternatus is one of the few Pokémon that retained Toxic after the TM was evicted and can use that to great effect here, taking advantage of its Titanic bulk to repeated switch in and badly Poison the target. Recover keeps Eternatus healthy over a long game and enabling it to take repeated hits. Venoshock and Dynamax Cannon are your STABs of choice. Venoshock eviscerates Fairies like Xerneas as well as synergizing well with Toxic, enabling Eternatus to hit hard even without investment, while Dynamax Cannon immediately pressures the many Dragons running around and punishes Dynamaxing to break through Eternatus. Mystic Fire rounds out the set, hitting Steel types Immune to Toxic and indirectly increasing Eternatus' Special Bulk.
Pressure synergizes incredibly well with this set, given the high number of low PP Signature moves that Legendaries are saddled with. Max HP and Defense to make Eternatus as bulky as possible -Mystic Fire will patch up the Special Defense.