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 player's subsequent battles against him.
The Rusted Shield does not appear as an item in the player's Bag until they've caught Zamazenta 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.