Spoiler warning: this article may contain major plot or ending details.
During the climax of Pokémon Shield, the player's Rusted Shield, along with Hop's Rusted Sword, summons Zamazenta and Zacian to help the two Trainers to battle against Eternamax Eternatus at the Tower Summit. The Rusted Shield temporarily becomes Zamazenta's held item for the duration of this battle. During the post-game, the player returns the Rusted Shield to Slumbering Weald. It is soon after temporarily stolen by Shielbert, before the player defeats him in a battle and retrieves the stolen item. Before the player battles Zamazenta at the Tower Summit in order to catch it, they give the Rusted Shield to it as its held item.
In Pokémon Sword, the Rusted Shield is instead obtained by Hop. During the post-game, when he returns it to the altar at Slumbering Weald, it is stolen by Shielbert. Hop battles Shielbert, but is defeated, allowing him to get away with the item. Later, Shielbert uses the Rusted Shield to lure Zamazenta to the Tower Summit. After the player has battled Zamazenta, it flees, and Hop retrieves the Rusted Shield from Shielbert, before chasing after Zamazenta. After Hop has caught Zamazenta at Slumbering Weald, the Rusted Shield becomes its held item in the subsequent battles he participates in.
In JN044, Goh and Sonia found the Rusted Shield at the altar in the Slumbering Weald after requesting the help of Zacian and Zamazenta in dealing with the Darkest Day. Goh collected it and the Rusted Sword and took both items with him. In JN045, the Rusted Shield called forth Zamazenta to the Tower Summit, who grabbed the Shield in its mouth, allowing it to transform into its Crowned Shield form. After Goh caught Eternatus, Zamazenta left its Shield behind. Ash and Goh were later seen returning the Rusted Sword and Shield where they once rested.
The Rusted Shield first appeared in a flashback in PASS05, along with the Rusted Sword. One year before the events of the Sword & Shield arc, Sōdo, accompanied by Shirudomiria, entered Slumbering Weald in order to see the two legendary artifacts. After confronting illusions of Zacian and Zamazenta at the altar at the end of the forest and convincing them they weren't trying to steal anything, the two Trainers approached the two artifacts on the altar, but were then knocked out by a bright burst of light. When they recovered, the artifacts were gone.
Trivia
The Rusted Shield does not appear as an item in the player's Bag until they have caught Zamazenta while it is holding it. This is likely done in order to prevent the player from removing the item from their Bag too early and contradicting the events of the game's story.
