Few Pokémon are as iconic in Gen IV of Pokémon as Dialga and Palkia. These are the legendary and box Pokémon for Pokémon Diamond, Pearl, and their remakes. Dialga and Palkia remain important in Pokémon Legends: Arceus as they gain a new form that players can change.
To change Dialga's form in Pokémon Legends: Arceus, you will to first complete the main game. After completing it, you will receive Request Number 89, The Diamond Clan's Treasure. Speak with Adaman, the leader of the Diamond Clan who can be found in the Obsidian Wastelands. He will challenge you to a battle.
Once you defeat Adaman, he will give you the Adamant Crystal. Use this item on Dialga and it will change forms. You can remove the item to change its form back, allowing you to change them freely. It should be noted that the new form is called the Origin Form, while the one players are used to is the Altered Form.
This is a neat quest to take on after having completed the game and a good way to keep Dialga relevant to the scene. If you want to get Dialga's Origin Form, prepare for a fierce battle against Adaman!
For more information on Pokémon, check out Best Nature for Pichu, Pikachu, and Raichu in Pokémon Legends: Arceus and The best early-game Pokémon in Pokémon Legends: Arceus on Pro Game Guides.