After President Rose interrupted you and the Champion Leon before battling in the last match of the tournament, he announced how he would trigger a new Darkest Day so that Galar could reach its apex of glory. You obviously couldn't stand for this and infiltrated Hammerlocke's tower, defeated President Rose, then got to the roof of the tower. After Leon failed to catch Eternatus, the Pokémon that Rose used to trigger the Darkest Day, it transformed into its perfect form:

Eternamax Eternatus.

You and your rival Hop try to fight it off and fail as well, which ends up with Eternatus knocking you, Hop and Leon out of the tower and back down into the city. Miracolously, you survived the fall thanks to your Pokémon and the panicked state Hammerlocke was in already hinted at what's happening around the entire region of Galar: citizens are panicking while various Pokémon suffer the effects of Dynamax and Gigantamax, losing control of themselves and wreaking havoc across the land.

Now more than ever, only the legendary wolf Pokémon Zacian and Zamazenta could potentially help, but both you and Hop lost the Rusted Sword and Rusted Shield when Eternatus made you fall. And for what you know, they could be anywhere...

... You have to get the Rusted items back, whatever the cost may be.