Zamazenta is one of three Legends found in The Galar Region, specifically being the Cover Legend of Pokemon Shield. Since it's exclusive to Pokemon Shield, it can't be found in Pokemon Sword. Instead, those playing Pokemon Sword will only be able to obtain Zamazenta by trading with someone who owns Pokemon Shield.
For those playing Pokemon Shield, Zamazenta can, in fact, be captured. However, unlike Cover Legendary Pokemon of the past, Zamazenta can't be captured until a player has finished the Main Story and has become the newest Galar Region Champion. After doing so, the Post Game will begin. After dealing with Sordward and Shielbert and their antics, players will eventually be able to battle both Zacian and Zamazenta near the conclusion of the Post Game. Since Zacian is the Cover Legend to Pokemon Sword and is exclusive to that game, it won't be able to be captured when battled. Zamazenta, on the hand, can be captured.
When battled at the summit of the Energy Plant, Zamazenta will be at Level 70 and in its Crowned Shield Form (since it will be equipped with the Rusted Shield). Since it will be in its Alternate Form, Zamazenta will be a dual Fighting / Steel Type (making it weak to only Fire Type, Fighting Type, and Ground Type Attacks). Zamazenta will know the moves Iron Defense, Slash, Crunch, and Behemoth Bash, and it will have its Defense Stat raised by one stage when the battle begins due its Dauntless Shield Ability.
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