Walkthrough:
With Sordward & Shielbert off with the one Rusted Artifact (with Hop going after them), you and Sonia will return to her Lab in Wedgehurst (where you'll also meet her new Assistant). Sonia will mention how her Power Spot Detector went off when the two arrived back in the Slumbering Weald - meaning they must have Wishing Stars with them - so she might be able to track down Sordward and Shielbert by using it. When she does, she'll notice a high concentration of particles at the Turrfield Gym. When there, you will find both Milo and Piers.
Apparently, they were having an Exhibition Match when a Pokemon just started to Dynamax out of nowhere. You will then have to team with the two, along with Hop, to take down the Dynamaxed Pokemon in a Max Raid Battle. The Dynamaxed Pokemon will be a Level 62 Tsareena, one that will know the moves Power Whip, High Jump Kick, Play Rough, and Zen Headbutt (also being able to use the Max Moves associated with each of those attacks).
After defeating the Dynamaxed Tsareena, it will calm down and you will receive Milo's Rare League Card. Sordward and Shielbert will then appear and say that they were the ones that Dynaxmaxed the Tsareena and that they intend to keep doing so with other Pokemon. Sonia will then arrive to tell you that both the Hulbury Gym and Motostoke Gym are now under Attack by Dynamaxed Pokemon.