Regigigas is potentially the ultimate Regi Pokémon, appearing in Brilliant Diamond and Shining Steel to let the others know there is a new king in town. While you can get Regirock, Reisteel, and Regice, Regigigas is the most powerful of the bunch.
This Normal-type Legendary has great move variety, meaning he can be quite adaptable depending on what you need in your group of Pokémon.
Regigigas is a post-game Pokémon, so you will need to have finished the story, deleted the Elite Four, and have access to the National Pokedex. You’ll also need to have access to Ramanas Park on Route 221. Head for the park and then you’ll need to exchange Mysterious Shards for three Discovery Slates. You can find Mysterious Shards in the Grand Underground. Bring these to the Discovery Room and then you will need to capture Regice, Registeel, and Regirock.
Make sure all three of them are in your party, then leave Ramanas Park and make your way to Snowpoint City in the very north of the map. Go to the Snowpoint Temple behind the Pokémon center and head inside. Go downstairs and when you reach the very bottom you will encounter Regigigas. Regigigas is a tough fight but will be weak against Fighting-type, so try and fill your available slots with strong options from that type.
