This is an unconfirmed theory that I believe to be true, I might post more of these. Or I might not. Who knows.
So Galar had two twin brothers, who -- at the age of 20 -- experienced the first Darkest Day 3000 years ago as Eternatus crawled out of its meteor. By teaming up with Zacian and Zamazenta, along with wielding the Rusted Tools, they were able to drive off Eternatus. They went down in Galar's history as heroes and were made Galar's first kings. That all is confirmed.
But there's another legend who has similar characters -- namely Unova's legend of two twin princes. I think the two Galar heroes and the two Unova heroes are the same people. If you think about it, it wouldn't make sense for Unova -- a region without a monarchy -- to have KINGS, meaning they would have to have been from another region that DID have a monarchy -- like Galar.
I believe that the two kings stayed in Galar long enough to have wives and children (since we know Sordward and Shielbert are part of that royal lineage which stayed in Galar those 3000 years). But after that, the two brothers moved to Unova to found that region too.
This leads us to the Unova legend, in which during their founding, they teamed up with a dragon. Later, they could not agree on which value was more important -- ideals or truth. They grew more and more argumentative, until the dragon split into two. Reshiram went with the elder brother who sought truth, and Zekrom went with the younger brother who sought ideals. Kyurem was also created from the empty shell, but it's not important to this theory.
The two brothers warred for a time, before Reshiram and Zekrom decided it would be for everyone's good if the two of them sealed themselves inside bodies of stone. But an uneasy peace isn't the same as a truce, meaning the two brothers were likely still angry with each other. But the rest of Unova's founding was successful, so how could that have happened with two warring brothers?
Obviously, one of them left for a different region, leaving the other one to found Unova. Since Unova's founding was successful and peaceful, I believe that the "good" brother and the "good" dragon, Reshiram stayed to found Unova, while the younger brother left for a different region.
That region is Kalos. If you think about the timelines, the Darkest Day happened 3000 years ago, and so did the war in Kalos. I think the younger twin is AZ, and he waged war on another land (Paldea, most likely) because of his bitterness with society. He sent his beloved Floette to participate in the war, and though we don't know which land won, the war ended and Floette was returned to AZ dead in a tiny coffin.
AZ was already bitter, but this pushed him over the edge. He created the Ultimate Weapon and stole the life forces of every human and Pokémon in Kalos, giving them all to Floette to resurrect it. Though it worked, Floette saw the destruction that AZ caused and left him.
AZ was now completely alone. Everyone else in Kalos was dead, his brother was in a different region, and his only friend had just left him. Eventually, Pokémon and humans began to move and migrate to Kalos, repopulating it, and because of the overload of life force, AZ -- now immortal -- was forced to watch the devastating consequences of his actions.
There we go, that's the theory. These three legends are connected. I'm convinced. Anyways, if there's any comments you guys want to make, maybe if I got something wrong, or if you think something different happened, feel free to let me know