Walkthrough:
After your battle against Hop, Sonia will arrive and gift you with the book she wrote about the true Legendary Heroes of The Galar Region. She will then suggest to both you and Hop to return the Rusted Sword & Rusted Shield to the Altar. Directly after you do, however, Sonia's Power Spot Detector will go off as two people arrive. These two people introduce themselves as Sordward and Shielbert, the descendants of the first kings of the Galar Region.
They will claim that Sonia's Book is filled with nothing but lies, and then the two will take the Rusted Sword & Rusted Shield. Both Hop and your trainer will attempt to stop them from doing so, but in order for them to hand the artifacts back over, you will first have to defeat them in back. If you're playing Pokemon Sword, you will battle Sordward. If you're playing Pokemon Shield, you will battle Shielbert.
Pokemon Trainer Sordward: Level 60 Sirfetch'd, Level 60 Golisopod, Level 60 Doublade, Level 60 Bisharp
Pokemon Trainer Shielbert: Level 60 Sirfetch'd, Level 60 Bronzong, Level 60 Falinks, Level 60 Klinklang
Once you have won your battle, you will receive the item back that you originally had during the Eternatus Max Raid Battle. Unfortunately, Hop will have lost his battle, and the two royal descendants will make off with it with Hop in pursuit of them.