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.
The Rusted Shield appeared in a flashback in The Champion, under the possession of Hop. When he held it up during his and Victor's battle against Eternatus at the Tower Summit, it summoned Zamazenta to their aid and allowed it to transform into its Crowned Shield form.
The Rusted Shield first appeared in a flashback in Ooh Shiny!! Legendary Equipment, along with the Rusted Sword. One year before the events of the Sword & Shield chapter, Henry, accompanied by Casey, 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.
In PASS20, it was revealed that the artifacts had been stolen by Sordward and Shielbert. In PASS26, they used the artifacts to "prove" themselves as descendants of Galar's ancient heroes. When the two brothers took on Eternatus at the Tower Summit, they took out the artifacts, but nothing happened, and the items were sent flying to the moat surrounding the Hammerlocke Stadium, from which they were recovered by Casey's Falinks and Stunfisk and taken to her, who gave them to Henry to clean and repair. However, to his horror, Henry soon realized that he could not forge and repair the artifacts. In PASS30, the artifacts, along with Henry, disappeared as Zacian and Zamazenta attacked Eternatus, getting teleported away by an unseen Pokémon. Casey later discovered that the artifacts had wound up in two different places: the Isle of Armor and the Crown Tundra.
In PASS37, the Rusted Shield was revealed to have been found by Galarian Moltres and hidden at Loop Lagoon. Henry was able to reclaim the artifact from Moltres shortly before it grabbed him and took him to the Crown Tundra.
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.
