Zacian is one of three Legends found in The Galar Region, specifically being the Cover Legend of Pokemon Sword. Since it's exclusive to Pokemon Sword, it can't be found in Pokemon Shield. Instead, those playing Pokemon Shield will only be able to obtain Zacian by trading with someone who owns Pokemon Sword.
For those playing Pokemon Sword, Zacian can, in fact, be captured. However, unlike Cover Legendary Pokemon of the past, Zacian 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 Zamazenta is the Cover Legend to Pokemon Shield and is exclusive to that game, it won't be able to be captured when battled. Zacian, on the hand, can be captured.
When battled at the summit of the Energy Plant, Zacian will be at Level 70 and in its Crowned Sword Form (since it will be equipped with the Rusted Sword). Since it will be in its Alternate Form, Zacian will be a dual Fairy / Steel Type (making it weak to only Fire Type and Ground Type Attacks). Zacian will know the moves Swords Dance, Sacred Sword, Crunch, and Behemoth Blade, and it will have its Attack Stat raised by one stage when the battle begins due its Intrepid Sword Ability.
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