Eternatus, also known as The Darkest Day, is an incredibly powerful and evil legendary pokemon. It is a poison and dragon type, and one of the three main legendaries from Pokemon Sword and Shield. It is the arch-nemesis of the other two legendary pokemon, Zacian and Zamazenta. It is the source behind the dynamax/gigantimax phenomenon. The story goes that in the past, a terrifying event called The Darkest Day forced pokemon to grow giant sized and rampage uncontrollably. Until one day, the legendary Sword and Shield put an end to The Darkest Day. The truth behind the story has varied over the years. However, the player learned that the Darkest Day is really Eternatus and the Sword and Shield are really Zacian and Zamazenta. Unfortunately, a man named Chairman Rose wanted to use Eternatus to generate limitless Galar particles (the stuff that causes pokemon to dynamax) as a form of unlimited energy, not realizing that Eternatus is pure evil and only cares about the destruction of the Galar region. But the player, along with his best friend, Zacian and Zamazenta, manage to defeat Eternatus, allowing the player to catch it. Now on the side of good as one of the player's pokemon, Eternatus helps the player become the new champion, and helps with whatever the player needs to do.