Arceus knew that the void should not stay a void. But instead of creating the multiverse by itself, with the power of Unowns, it shaped 3 Pokemon, Dialga-the Temporal Pokemon, Palkia-the Spatial Pokemon and Giratina-the Renegade Pokemon. These 3 first created their home dimensions, the Temporal World, the Spatial World and the Distortion World respectively. These worlds formed time, space and anti-matter, hence the 3 Pokemons were seen as guardians over the elements.
Just to make sure you lore-seekers are still on the same page, these 3 Pokemon all have 2 forms. Their Origin Form works best in their home dimension while their Altered Form is used when they are outside of their dimensions. They all needed 2 forms for different adaptations towards the different environments. By the way, changing the forms only change how their powers are spread but don't change how much power they have. It's complicated... I know.
Arceus has a home dimension too! It is said when a specific flute with specific notes is played at the top of Mount Coronet- the Spear Pillar, stairs will appear and bring you straight to where Arceus watches over all of us. Although only 3 people were able to reach there that is... An ancient hero believed to have been one of the old Celestica People, the saviour in the "Coronet Crisis" and the 10-year-old who was able to defeat Team Galactic on the news.
I should also talk about the "Multiverse Theory". It is said when the Creation Trio formed time and space, what's been made isn't just our universe, but instead, unlimited amounts of them, with each can be having tiny differences or significant differences from the other. If we ever meet these legendary Pokemon, the theory suggests that this Pokemon we are seeing is just a part of what they truly are, with unlimited amounts of these "parts" guarding the multiverse.
Dialga, Palkia and Giratina had also stored some of their power in certain crystals. It is said that once they are activated, they can change the forms of the Pokemons and boost their power at the same time! They are the Adamant Orb for Dialga, the Lustrous Orb for Palkia and the Griseous Orb for Giratina.
Arceus, as the master of the three, watches over and only interrupts their actions only when necessary. But it seems that Arceus takes the word "necessary" to the extreme! For example, Arceus has only appeared twice in the myths we are currently known to - The "Coronet Crisis" we just talked about and the "Giratina Rebellion"... that's another story.
Anyways, the Creation Trio created the universe! Great! But life isn't here yet...
