There is a lot of content in Pokemon Legends: Arceus, and you can get items from completing the story. The finale of the story, however, involves a battle against Dialga or Palkia. The Pokemon you will face will depend on the choices you have made in the story.
You can only get Dialga and Palkia on the final boss fight of the story. Siding with the Diamond Clan will cause you to fight Palkia. On the other hand, siding with the Pearl Clan will cause you to fight Dialga. Eventually, you will be able to catch both Pokemon.
Dialga is weak to Pokemons with fighting and ground-type moves. On the other hand, Palkia is weak to a dragon and fairy-type moves. Keep in mind that you will face them as level 65, so make sure you are well prepared to fight them.
Once you get both Pokemons and finish the entire game, you can change the form for Dialga and Palkia.
NOTE: You can only get the items for changing their form after you have finished and completed the game.
After you complete the game, you will receive new requests from both clans. You need to complete the requests “The Diamond Clan’s Treasure” and “The Pearl Clan’s Treasure”. Once you complete both quests, you will get a Lustrous Globe and Adamant Crystal.
Use these items on Dialga and Palkia to change their forms.