In order to catch Legendary Pokémon other than Dialga or Palkia in Brilliant Diamond and Shining Pearl, you'll need items called Mysterious Shards.
These small stones are only found in the Grand Underground, and will allow you to obtain Slates in Ramanas Park.
Where to find Mysterious Shards
Once you've undertaken the Underground Man's training missions, you'll unlock the Hammer and Pickaxe.
The second condition is having beaten the Pokémon League and completed the Regional Pokédex. This unlocks Ramanas Park, where you can catch Legendaries, as well as making Mysterious Shards appear when digging for treasure in the Grand Underground.
While digging, you'll come across Stone Boxes. These contain statues you can display in your Secret Base. Other items you can dig up include Evolution Stones, Spheres, and even Fossils.
Among all these treasures, there's a chance to uncover either Mysterious Shard L or Mysterious Shard S.
To optimize your chances of finding these shards, you can either:
- Enter the Grand Underground in a multiplayer mode, as both treasure found and Digletts amassed count for both players;
- Amass 40 Digletts to increase your chances of finding Mysterious Shards significantly.
