Legendary Pokémon from previous generations are available in Brilliant Diamond and Shining Pearl, with most able to be obtained in just one version of the game.
In order to find them, you'll need to head to Ramanas Park. However, there are a few prerequisites:
Which Slate do you need to find Groudon?
Ramanas Park is a vast area made up of several caves, in which a specific type of Slate is requested.
In order to unlock Groudon, take the Tectonic Slate to the Tectonic Room and place it on the plinth.
To unlock this slate, you'll need to have caught all three Legendary Birds or all three Legendary Beasts. Check out the link at the bottom of this guide for Slate prerequisites.
Save your game before placing the Slate, so that you don't accidentally knock out Groudon in the ensuing battle.
Meet Groudon, the Continent Pokémon
Capturing Groudon can be challenging. Here's what you need to know about this super-ancient Pokémon.
Groudon's shiny form is easily distinguishable from the base form, being a lime green color, so you shouldn't miss it if you're looking to add the rare form to your collection.
Tips to make it easier to catch Groudon
Follow this advice to make catching Groudon easier:
Use a Quick Ball at the very start of the encounter, and a Timer Ball if the battle lasts more than 10 rounds.
Take a Pokémon with False Swipe to ensure you don't accidentally kill it.
Take a Pokémon that can inflict Paralyze or Sleep.
Pay attention to how much PP its moves have.
Save your game right in front of it, before engaging.
Be mindful that Groudon can heal itself with Rest.
Watch out for Precipice Blades, a devastating attack which could knock you out if you're underleveled.
Scyther is a great option thanks to its dual Bug/Flying types, and the fact it will only take damage from Groudon's Hammer Arm move.
If you accidentally knock it out, you will need more Tectonic Slates. Once you have captured it, you will not be able to re-summon it.
Additionally, upon exiting, an NPC will give you a Groudon Statue for your Secret Base.
You'll also be able to pick up some Berries.