Eternatus

Eternatus is a Poison/Dragon-type Pokémon introduced in Generation VIII. It serves as the final antagonist of Pokémon Sword and Shield.

Biography
Eternatus is an alien that appeared 20,000 years ago when the meteorite it was riding impacted the Pokémon planet. To recharge itself, Eternatus absorbed everything it could harness within its core until Zacian and Zamazenta, the guardians of the Galar region, defeated the monster and sealed it away. From within its imprisonment, Eternatus gathered energy across the Galar region to heal itself from the core on its chest. When Pokémon come in contact with Eternatus' energy, they enlargen in size, a term referred to as Dynamaxing.

Chairman Rose would awaken Eternatus from its slumber to inherit its power for himself within the Energy Plant. But because Eternatus was not fully healed of its wounds, it grows feral and destroys everything in its path like it did thousands of years ago. Champion Leon tries to catch Eternatus to calm it, only for it to break out of the Pokéball. Leon then orders the player to fight and defeat Eternatus, which they manage to do. But this only angers Eternatus further, making it absorb all remaining energy within the Galar region, causing Pokémon to enlarge berserkly.

Eternatus transforms into its own Gigantamax form called "Eternatus Eternamax", where the player and Hop were completely unable to command their Pokémon due to Eternatus' interferences. Suddenly, Hop remembers about the Sword and Shield relics, and uses their powers to awaken Zacian and Zamazenta. The relics return the two to their true forms, restoring enough energy to let the player and Hop defeat Eternatus. The player then throws a Pokéball and manages to capture Eternatus, soothing its wrath and restoring Galar back to normal.

Trivia

 * Eternatus is the third Pokémon to have a Poison/Dragon typing, after Dragalge and Nagandel.
 * Eternatus is the only Pokémon seen in the series so far that tries to resist entering a Pokéball.
 * Should the player somehow be out of Pokéballs when trying to catch Eternatus, Hop gives some of his to them.
 * Due to it being from space and its eldritch appearance, some speculate that Eternatus is an Ultra Beast, although this rumor has not been confirmed.