According to the legends, before all started, Arceus originated from an egg that appeared from the primordial void, emerging ans creating the Pokémon of myth and the entire Pokémon universe. Known as the Original One, Arceus was worshiped in ancient times by the people of Hisui under the name "Almighty Sinnoh"
According to legend, it shaped the Pokémon universe using its 1,000 arms. Generally, in myths and legends and is considered one of the strongest Pokémon. It also created eggs which the Pokémon of myth (Dialga, Palkia, and Giratina) hatched from, as well as another egg which the lake guardians (Uxie, Azelf, and Mesprit) hatched from. It could also recreate these eggs if it wanted to. Because of this, Arceus is thought to be one of the longest-lived Pokémon species, if not the oldest Pokémon in existence.
Arceus does not suffer fools gladly and will attack in a violent rampage if betrayed or deceived. It also holds long grudges if betrayed by those it considered friends. It can release countless shots of light that disintegrate the landscape around it. Arceus can change its type at will to adapt to a situation through the use of its well-knowed Plates or type-specific Z-Crystals.
Arceus was known to care for and safeguard the planet in ancient times, protecting the world from cataclysms such as meteors, and it would show gratitude and compassion to those who treated it with kindness. In modern times, Arceus continues to watch over the world and wishes for people and Pokémon to work together in the face of adversity. Even when it is needed, it only gives hints on how to solve problems instead of directly interfering. Arceus can also give its blessing to other Pokémon through bolts of light, making them bigger and more powerful, as seen with the ancient hero's 10 Pokémon companions. This blessing is passed down through family lines, as seen with the descendants of the hero's Pokémon: the noble Pokémon and ride Pokémon worshiped by the people of Hisui.
Also, just a funny fact, in the ancient Hisui, Palkia and Dialga were often mistook about being the "Mighty Sinnoh" by the Pearl and Diamond Clan respectively
But there's a little fact that not much people knows, This Pokémon is merely an extension of the real Arceus, whose true form is unknown. Just an avatar or a manifestation of the true, all-powerful Arceus, who exists as a being far beyond the confines of the physical world. This allows the true Arceus to remain in a higher dimension and avoid this world being destroyed by its full power, which is beyond comprehension.
You had completed the entire Pokédex and, in turn, had obtained the Azure Flute. You knew what you had to do. You headed to the Spear Pillar/Temple of Sinnoh (This depends on whether you want to be in the present, in Sinnoh, or if you want to be in the Past, in Hisui, respectively).
You played the flute and after a few seconds, a glowing staircase appeared, leading to... the sky? You climbed the stairs. As you climbed, the surroundings turned as dark as the night sky, and the stairs were surrounded by a purple aurora.
You reached the end, on a platform consisting of a large, circular floor with blue light circles and rays of purple light visibly surrounding the platform. You walked to the center of the platform and started looking around, something felt out of place... like something was missing...
A shiver runs down your spine as you feel a presence behind you, you quickly turn around and... there it was, the Almighty Arceus standing in front of you, watching you intently.