From Bulbapedia, the community-driven Pokémon encyclopedia.
The Darkest Day (Japanese: ブラックナイト Black Night) is a black storm that covered the Galar region 3,000 years prior to the events of Pokémon Sword and Shield. During the course of the story, Rose deliberately causes another occurrence in an ill-advised effort to replenish Galar's Dynamax energy.
3,000 years ago, when Eternatus attempted to stay alive by absorbing Galar's energy, it caused a red light to appear. This resulted in the Darkest Day. This phenomenon, in the form of a black storm, caused Pokémon to Dynamax and Gigantamax. However, they went berserk in the process, almost destroying the region. The event ended when Eternatus was defeated by Zacian and Zamazenta.
In the present day, Rose aims to use Eternatus to bring about a second Darkest Day above Hammerlocke Stadium to avert the Dynamax energy shortage predicted to happen after a thousand years. Sonia has also conducted research into the Darkest Day. She learns that the event was connected to one Pokémon and that the two heroes of Galar were not humans, but rather Pokémon.
Rose awakens Eternatus during the time the player and Leon are about to have their Championship Match. However, he could not control it, so Eternatus caused the Darkest Day to happen again, affecting all the Power Spots in the region. When the player defeats Eternatus for the first time, it absorbs all of the energy in Galar to achieve its Eternamax state, further speeding up the Darkest Day. The player and Hop then bring out the Rusted Sword and Rusted Shield with the hopes of stopping Eternatus. This results in Zacian and Zamazenta awakening and coming to their aid. The four work together to defeat Eternatus in a Max Raid Battle before the player catches it, thus putting an end to the second Darkest Day.
Hammerlocke vault tapestries
The Hammerlocke vault contains a series of tapestries describing the Darkest Day, which are shown from the perspective of the two heroes who stopped it.
...a mysterious black storm incoming...
...two heroes uses Sword and Shield to fight...
...and became crowned as kings.
The Bob's Your Uncle restaurant in Circhester contains a ripped part of tapestry, presenting two heroes standing before two tombstones with sword's and shield's symbols on altar similiar to that one in Slumbering Weald.
In the anime
In Toughing It Out!, Sonia told Ash and Goh the story of the Darkest Day.
In JN044, the Darkest Day began again after Eternatus emerged from the Energy Plant core. Rose also revealed that "Darkest Day" was what people used to call Eternatus, before he gave it its current name.
In JN045, Leon battled Eternatus but failed to catch it. As it assumed its Eternamax form, Ash and Goh started battling it, but they were outmatched as well. However, using the Rusted Sword and Rusted Shield that Goh had found in the Slumbering Weald, they were able to summon Zacian and Zamazenta to their aid and defeat Eternatus. Goh then caught Eternatus, ending the Darkest Day. Eternatus's Poké Ball was subsequently sealed away in an underground safe to prevent the disaster from occurring again.
In other languages