At Hammerlocke Stadium, ride the elevator down to the Energy Plant, then proceed west along the corridor, like the last time you were here. After you go down the slope, you'll catch up with Sordward or Shielbert and their cronies. After the buffoon has explained his nefarious scheme, speak to him to face him in battle.
Pokémon Trainer Sordward (Sword)
Sordward has the exact same party as his first battle in the Slumbering Weald, so if you could defeat him once, you can defeat him twice. But just in case, use Flying or Fairy-types for Sirfetch'd, Flying or Electric for Golisopod, Fire, Ground or Dark for Honedge and Fighting, Fire or Ground for Bisharp.
Pokémon Trainer Shielbert (Shield)
This is exactly the same fight as in the Slumbering Weald, but his Pokémon are higher Level. As a reminder, use Flying or Fairy-types against Sirfetch'd and Falinks, Fire, Ghost or Dark for Bronzong and Fire, Ground or Fighting for Klinklang. Unlike your battle against Rose here, you cannot Dynamax or Gigantamax.
Once the dust has settled, go up the nearby elevator to reach the tower summit. There, you'll find the Legendary Sword or Shield Pokémon, except it's gone berserk and needs calming down. You're the Champion so you're the best person for the job!
Similar to your initial encounter with Eternatus, you only have to defeat the Legendary Pokémon--you cannot catch it right now or ever (unless you've got the other version of the game). The Legendary Pokémon will be in its weaker non-Crowned form, but you should exercise caution regardless.
If playing Sword, you'll fight Zamazenta, the Shield Legendary. It can use Iron Defence to greatly boost its Defence. Otherwise, it'll try to pummel you with Play Rough, Iron Head and Crunch. None of these are STAB moves, but they can still hurt. Use special attackers if possible, so you can ignore its Defence.
On Shield, you'll face Zacian. It likes to use Swords Dance to greatly boost its Attack. For attacks, it can use Iron Head, Crunch and Sacred Sword. None of these are STAB moves, but with its natural high Attack and potential Swords Dance boost, they will probably hurt a lot. Use Steel or Poison-types if you can.
After defeating the berserked Legendary Pokémon, it'll flee into the distance. Hop will pick up the rusted weapon that was stolen, before giving chase. All that's left to do is catch the other Legendary Pokémon that remains! Interact with the Legendary Pokémon when you're ready to begin.
This is the moment you've been waiting for. You can attempt to catch the Legendary Pokémon that belongs to your version. It'll be in its powered up Crowned form, so be extremely cautious. By the way, if you've got no plans for it, you could consider using the Master Ball for a guaranteed capture. Otherwise, let's do this!
Zacian, the Sword Legendary, has outstanding Attack and impressive Speed, plus decent bulk as well. It may use Swords Dance to greatly boost its Attack or attack with Sacred Sword, Crunch or Behemoth Blade--you should recognise the last one from the Eternatus raid battle.
Needless to say, Zacian is a beast. Nevermind weakening it, you should find a way to protect yourself first! Good methods to weaken it include paralysis (eg. via Nuzzle), burn (eg. via Will-O-Wisp) or rotating in Pokémon with Intimidate. Those that resist Steel, like Fire, Water or Electric-types are strongly recommended.
Zamazenta, the Shield Legendary, has brilliant Defence and Special Defence, but its Attack and Speed are very respectable as well. It can use Iron Defence to further improve its Defence or attack with Slash, Crunch or Behemoth Bash--the last one does major Steel-type damage.
Because of Zamazenta's incredible bulk, you can probably hit it with your strongest moves straight away, then gradually choose weaker ones. Although it's more defensively orientated, it can still hit really hard. If you've got some, Pokémon that resist Steel, like Fire, Water or Electric-types would be extremely welcome here.
When you catch Zacian and Zamazenta, they will automatically be in their Crowned Sword or Shield form. In this form, they gain the Steel-type and Iron Head changes into Behemoth Blade or Bash.
In order to maintain this form, your Legendary Pokémon must hold onto the Rusted Sword or Rusted Shield. If you remove this item or give them a different item, they will return to their weaker form.
Succeed in catching the Legendary Pokémon and everything will be over... Oh, there is one more thing. Hop managed to track down the other Legendary Pokémon and needs your help ASAP. Quickly make your way to the Slumbering Weald. Before that, you may want to rest your Pokémon at a Pokémon Centre or by speaking to your Mum at Postwick.