The Great Blast

As Sadalla was observing the development of the dragons, magi, and monstrous creatures, he got a notification from his system that no one ever wants to hear.

[First Planet has reached the threshold - Main Quest 3 activated: Renaissance]

Details: A mass-casualty event has fundamentally altered the planet. As humans rebuild, they seek to understand what led to their situation, and make certain that it never happens again. You must guide them in such a way that they no longer seek such a bloody path again. 

Task 1: Discover the cause of the mass-casualty incident

Reward: Planet Coupon

Task 2: Rebuild the locations impacted by the mass-casualty incident and guide the people to never forget what happened.

Reward: Avatar Coupon

Task 3: Bring the eyes of the people to their past, and let them see what their mistakes nearly cost them.

Reward: Completely terraform a planet and fill it with all necessary organisms to flourish.

Task 4: All Major Civilizations have made contact with each other.

Reward: Main Quest 4 activation

Sadalla was not amused. He once again placed his consciousness within Zygarde and absorbed the memories of his avatar. He quickly discovered that there was not in fact just one mass-casualty incident, but several. In the Yamato Islands, the Leviathan, known to locals now as Kyogre, and the Behemoth, what they called Groudon, had been woken up by something, restored to their former primal states, and basically destroyed most of the region of Hoenn, as the locals who survived called it. They had quelled the two legends by calling upon the Sky Snake, Rayquaza, to once again teach the beasts a lesson.

Meanwhile, the island of Galar had been ruthlessly shattered by some kind of extradimensional entity that caused a bunch of Pokémon to grow gigantic and cause a ruckus. Two giant wolf Pokémon had managed to defeat the entity, but it was only sealed away, not dead, and it had left a lasting impact upon Galar, with that gigantification aura still present across the island.

The biggest impact, however, had certainly been felt just south in Kalos. Rocked by years of wars between the Franks, Goths, Germans, and Lombards, the king of a minor region called Normandy named AZ had built a massive machine to restore his lost Pokémon, Floette, to life. Floette had been slain in the endless wars against the various tribes as the Franks slowly defeated the Goths and Lombards. The machine had successfully restored his Floette to life, but it had come at a steep cost. The machine had required the life energy of nearly one thousand Pokémon to be used, and the weapon, in a kind of sick irony, had made Floette and AZ immortal, and incapable of dying of old age.

In his sadness, however, AZ didn't even notice his curse, as he transformed his miracle machine into an Ultimate Weapon. The ultimate weapon quashed the Franks, Goths, Germans, and Lombards alike. Millions of beings were simply erased from existence. The blast's ripple effects had torn space asunder, and resulted in meteor strikes across the globe, and portals to other worlds opening, nearly at random.

That was what caused Groudon and Kyogre to awaken. It led to Eternatus's rampage in Galar, Giratina's escape that ended the Celestica Civilization, which had previously been able to handle a similar situation, and caused Wormholes to appear over Alola, permanently connecting the region to those places via a tear at the peak of a plateau. The energy from the tear mingled with the new meteor shower, forming Z-Crystals out of rocks filled with pure natural energy.

In Kalos and Hoenn, it also led to the emergence of Mega Stones and Key Stones, as the meteorites absorbed the life energy of the deceased people and Pokémon killed by the ultimate weapon and the wrath of Kyogre and Groudon. Kyogre's power was absorbed by one, forming the Blue Orb, and Groudon's formed the Red Orb. Upon this discovery, both had been sealed away in an altar to the dead, and the weapon that caused this was also sealed away by a megalithic graveyard called Geosenge. 

Sadalla also found out that this coincided with a war between two kings in Unova, who fought about what was more necessary between Truth and Ideals, causing the original dragon of the royal family to split into the two beings that reflected the pursuits of Truth and Ideals, Reshiram and Zekrom. The original dragon's remaining body became a hollow void of ice, representing the former king's pursuit of wholeness and unity between his two former princes.

Sadalla didn't know how the incidents were connected, but a couple of Pixie-like Pokémon had emerged from the meteors, and one of them, Victini, had indeed emerged in Unova. The other few, Jirachi, Manaphy, Phione, and Hoopa, had emerged in Hoenn, the ruins of Celestica, and Kalos, respectively.