You need to defect the Leon in the Champion Cup and become the new champ of the Galar region in order to move forward in the game.
If one player successfully defeated Leon the game will redirect you to the house form where the game starts. You need to meet Professor Magnolia will gift you a Master Ball after that player needs to exit the house and enter the Slumbering Weald from the left direction gate.
Now go to the location where you first get the rusted sword and shield in Slumbering Weald. There you encounter two brothers named Shielbert and Sordward.
You need to battle those brothers and defect them however your rival will lose to another brother. After both brothers started Dynamaxing Pokémon in Slumbering Weald. To fix this you head to Wedgehurst’s Pokémon Lab where Sonia can use the Power Spot Detectors to see the locations of the Dynamaxed Pokémons.
The player needed to travel to the seven different Pokemon Gyms and need to defect one Dynmaxed Pokémon in each gym and battle both brothers again. After some point, you need to visit the Hammerlock Stadium where you will see Zacian or Zamazenta on the Energy Power Plant.
Catching The Zacian & Zamazenta
In order to catch the legendary Pokemon player first need to defect the other version of Legendary Pokemon. This means first you need to defect the Zamazenta after that you can catch Zacian.
Once you defect the other version of the Legendary Pokemon you can then able to catch Zamazenta or Zacian.
Make sure to use the Master Ball that Professor Magnolia give you because it has a 100 percent catching rate. However, you can also use the Poke Ball if you are able to reduce the health bar of legendary pokemon enough.
There you have your Legendary Pokemon now you’re near to completing your Pokedex. If you want to catch more Legendary Pokemon make sure to get the Pokémon Sword and Shield Expansion Pass
This is all for this guide for more interesting content do read our Pokemon Go Guides for more helpful information.