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In pre-history, humans were just like pokémon. As they evolved into modern humans, they'd start setting apart from pokémon.
-Hisui- (then renamed Sinnoh, the original name given by humans to Arceus)
"the hero" of the Celestials is an ancestor of Alder which became possibly the first pokémon trainer in history when he tamed the noble pokémon struck by Giratina's "bolts of anger" and with them in his journey ended up reaching and battling Arceus. At this point, Arceus will take a liking in humans and will reveal the story of the creation of the multiverse and more to the hero, that will therefore carve it into Arceus's plates.
After this event, ¤Celestials will also build the divine shrines with the Original One's symbolisms, the spear pillar and the Sinjoh Ruins.
The history of the Celestials will end under mysterious conditions. Most likely, either their vicinance to Giratina or to Arceus will be the cause of the end of their prosperity, transporting them in an unknown place. After this, the remaining Celestials will tear down the statue of Giratina, that was put in a higher position than those of Dialga and Palkia similarly to how in the Sinjoh Ruins Giratina is the tip of the pyramid, signifying a more centered cult on Giratina than the other 2 brothers. While they'll be leaving intact the statues of Palkia and Dialga, they will even erase Giratina's name from history, myths and knowledge, and he'll basically become an unmentionable scare.
Yet, the vicinance of the Celestials to Giratina won't end, as both Cogita, Cynthia and Volo will relate/side and research about him again in more recent history.