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.
One of those is Regigigas, creator of the three Titans from Hoenn.
How to unlock Regigigas
To face Regigigas, you need to head to Snowpoint Temple. You'll need to have unlocked the National Pokédex first, by becoming Champion and then speaking to Professor Rowan in Sandgem Town to upgrade your Dex.
When done, return to the Temple and head down to the lower basement. When you interact with it, nothing will happen -- but don't fear, this is completely normal.
This is because there's another prerequisite -- catch its three creations, Regice, Regirock and Registeel, and have them in your active team.
These three Titans can be found in Ramanas Park.
Regigigas: Key Info
Make sure you're prepared to face Regigigas, as it'll prove to be a tough opponent. Here's the key details.
As with all other Legendaries, you'll be best served by Pokémon that can use False Swipe, as well as those who can inflict Paralyze and Sleep.
Additionally, starting the battle with a Quick Ball, using Timer Balls if the battle goes on past ten rounds, and Dusk Balls will make your job easier.
However, it's a Legendary, so you'll need to be patient. Scizor is a great Pokémon to use given its resistances against Regigigas. However, it won't inflict statuses.
Regigas Forms
Here are the two forms of Regigigas. Its normal form as well as its chromatic form.