This is a guide for Mission 18: “The Counterpart” in Pokémon Legends: Arceus (PLA).
How to Start Mission 18
You’ll automatically receive this mission when you return to the Summit Camp after completing the previous mission, “Atop Mount Coronet”. A second, overwhelmingly strong Legendary Pokémon has appeared, but you currently lack the means to stop it. After racking his brain for a bit, Professor Laventon has a solution.
1. Visit the Primeval Grotto
From the Summit Camp, make your way east-south-east towards the Primeval Grotto. You can quickly reach there by gliding with Braviary. Note that the south entrance of the grotto is guarded by an alpha Probopass; meanwhile, there’s a side entrance to the west that’s relatively safe.
Inside the grotto, make your way to the suspicious spot towards the east where the glowing red stones are. Before you can make a move, the Bandit Sisters jump in to stop you. Dude, we’re trying to save the world here…
Rhydon is easy pickings for Water or Grass-types. Ground and Fighting-types should do OK, but most are physical-based and Rhydon has high Defense. Steel-types are ill-advised due to Rhydon’s Ground moves.
After shooing away the unwanted guests, you’ll receive the Origin Ore that you were seeking.
2. Return to the Summit Camp
Fast-travel back to the Summit Camp and show Laventon the goods. You’ll be whisked back to Galaxy Hall at Jubilife Village. Akari/Rei will work their crafting magic and produce the Origin Ball–a one-of-a-kind Poké Ball that can ensnare the remaining Legendary Pokémon at the Temple of Sinnoh.
3. Head to the Temple of Sinnoh
Origin Ball firmly in hand, make for the Front Gate of Jubilife Village. You’ll bump into Cogita who offers some advice, before going to do some pre-apocalyptic shopping. Set out for the Coronet Highlands, choosing the Summit Camp as your start point. Like the previous mission, head north-east towards the Stone Portal and go inside.
Along the way, you’ll run into Volo, who’s way too excited about this kind of thing. On the other side of the Stone Portal, proceed up the slope towards the front of the temple. Adaman, Irida and Kamado will be waiting to give you the final go ahead.
4. Confront Palkia or Dialga
This is it: the final battle. Depending on which clan leader you chose at the Ancient Retreat, you’ll encounter Palkia (Adaman) or Dialga (Irida). However, this time, the Legendary somehow gains incredible power, transforming into a brand new form…
Upon defeating and catching the Legendary Pokémon, peace will finally return to the Hisui region. This mission–and the story–will come to a close. Now sit back and enjoy the credits (or go and make some coffee etc.)!