Spoiler warning: this article may contain major plot or ending details.
During the climax of Pokémon Sword, the player's Rusted Sword, along with Hop's Rusted Shield, summons Zacian and Zamazenta to help the two Trainers to battle against Eternamax Eternatus at the tower summit. The Rusted Sword temporarily becomes Zacian's held item for the duration of this battle. During the post-game, the player returns the Rusted Sword to Slumbering Weald. It is soon after temporarily stolen by Sordward, before the player defeats him in a battle and retrieves the stolen item. Before the player battles Zacian at the tower summit in order to catch it, they give the Rusted Sword to it as its held item.
In Pokémon Shield, the Rusted Sword is instead obtained by Hop. During the post-game, when he returns it to the altar at Slumbering Weald, it is stolen by Sordward. Hop battles Sordward, but is defeated, allowing him to get away with the item. Later, Sordward uses the Rusted Sword to lure Zacian to the tower summit. After the player has battled Zacian, it flees, and Hop retrieves the Rusted Sword from Sordward, before chasing after Zacian. After Hop has caught Zacian at Slumbering Weald, the Rusted Sword becomes its held item in the subsequent battles he participates in.
The Rusted Sword appeared in a flashback in The Champion, under the possession of Victor. When he held it up during his and Hop's battle against Eternatus at the tower summit, it summoned Zacian to their aid and allowed it to transform into its Crowned Sword form.
The Rusted Sword first appeared in a flashback in Ooh Shiny!! Legendary Equipment, along with the Rusted Shield. One year earlier, 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 Cracking!! Brilliant Tap-Dancing, it was revealed that the artifacts had been stolen by Sordward and Shielbert. In Cracked!! The Rusted Sword and Shield, 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 PASS39, Bede mentioned he had detected signs of the Rusted Sword on Spectrier. In PASS40, he detected signs of the artifact at Spectrier's barn at the Crown Shrine, but the artifact wasn't there. Once Calyrex had tamed its steed, it told Spectrier to take it to the artifact. After finding it, Calyrex brought it to Henry at the Crown Shrine, allowing him to purify it of Eternatus's poison. In PASS41, Zacian used the sword to transform to its Crowned Sword form.
The Rusted Sword was introduced as a Pokémon Tool card in the Pokémon Trading Card Game during the English Sword & Shield Series (the Japanese Sword & Shield Era). It was first released in the Japanese Shiny Star V expansion before debuting in English in the Shining Fates expansion, with artwork by 5ban Graphics. It allows Zacian V to deal 30 more damage to the opponent's Active Pokémon.
Trivia
The Rusted Sword does not appear as an item in the player's Bag until they have caught Zacian 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.
The Rusted Sword appears to feature fictional runic characters, likely a combination of both Japanese and Old Norse characters.
