Walkthrough
After defeating either Sordward or Shielbert, they will tell you that their brother has already had plenty of time to summon and experiment on Zacian/Zamazenta. When you head to the top of the Energy Plant, however, you will find that the Legendary Pokemon are angry with both Sordward and Shielbert. Of course, it will be up to your trainer to save the brother at the top of the Energy Plant by battling and calming down the Legendary Pokemon.
If you're playing Pokemon Sword, you will battle Zamazenta. If you're playing Pokemon Shield, you will battle Zacian. Both Pokemon will be at Level 70, and each will be in their Hero of Many Battles Form. Zamazenta will know the moves Play Rough, Iron Defense, Crunch and Iron Head, and, because of its ability, it will have its Defense boosted when the battle begins. Zacian will know the moves Sacred Sword, Sword Dance, Crunch and Iron Head, and, because of its ability, it will have its Attack boosted when the battle begins. You will not be able to capture said Legendary Pokemon within this battle.
After defeating the Legendary Pokemon and effectively calming it down, it will disappear back to the Slumbering Weald, and Hop will chase after it in hopes of making sure it stays this way. The other Legendary Pokemon that is exclusive to the version you're playing will then appear and challenge you to a battle, and by giving said Legendary Pokemon their Rusted Artifact to adorn, you will be able to capture it with this battle.
If you're playing Pokemon Sword, you will now battle Zacian. If you're playing Pokemon Shield, you will now battle Zamazenta. Both Pokemon will be at Level 70, and each will now be in their Crowned Form. Zacian will know the moves Sacred Sword, Sword Dance, Crunch and Behemoth Blade, and, because of its ability, it will have its Attack boosted when the battle begins. Zamazenta will know the moves Slash, Iron Defense, Crunch and Behemoth Bash, and, because of its ability, it will have its Defense boosted when the battle begins.
Once you have captured the Legendary Pokemon exclusive to your version, Sordward and Shielbert will realize the error of their ways, promising to make atonement for their crimes. Shortly after this, your Rotom Phone will begin to ring. It will be Hop, and he'll need your help with the Cover Legendary Pokemon.