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, Chairman 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 became berserk in the process, almost destroying the region. The event ended when Eternatus was defeated by Zacian and Zamazenta.
In the present day, Chairman Rose aims to use Eternatus to bring about the 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 that the two heroes of Galar were not humans but 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 the energy in Galar to achieve its Eternamax state, further speeding the Darkest Day. The player and Hop then bring out the Rusted Sword and Rusted Shield in hopes to stop Eternatus. This results in Zacian and Zamazenta awaking and coming to their aid. The four work together to defeat Eternatus before the player catches it, putting an end to the Darkest Day.
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