Adam and Eve
The two in past were together like glue, going everywhere, exploring everywhere. They were 2 balls of energy with seemingly infinite energy with an unsatiable search for entertainment.
However, based on what Arceus estimates. Around the time the 2 first met humanity they likely had different opinions on humanity, Adam who wanted to see their potential, and Eve who thought it was a waste of time watching them.
From there the two separated, rarely ever meeting with each other as they go about their own lives.
They might just be a simpler reason than a heated argument.
Adam and Eve got bored of each other.
Having spent millions of years together, this isn't impossible, especially after seeing human uniqueness and feeling their relationship was bland.
***
[6 years later...]
Location: XXX Hotel, Pewter City, Kanto.
"Happy Birthday!" Puff the Wigglytuff cheers hugging her master tightly.
The small room they were in was covered in sparkles, balloons, and other such birthday ornaments. Excluding Puff, there was a Rhyperior, Ancient Pidgeot, Ancient Houndoom, Starmie, and a Venusaur, all being Peak Guardian Pokemons.
She's more than strong enough to have become Kanto's champion, however, her age wasn't enough.
With her team of Pokemons all shouting 'Happy Birthday' to her, she froze in shock and disbelief. Despite having lived a full 10 years, this was the first time someone had told her this.
"Why didn't you tell us you humans consider birthday important?" Puff scolded. "Worst of all, I bet he knew yet but never bothered to..."
"Puff, keep father out of this," Tatsumaki said. "Besides, this is a huge waste of time. Now, that I'm 10, we're going to use every second to accomplish my goal,"
"We can go after the celebration, but for now you have to enjoy yourself," Puff declares. "If you're not satisfied with the presentation, do let Rhy and Venus know,"
"Um," Rhy the Rhyperior nods while giving Tatsumaki a thumbs up.
"We've tried our best," Venus the Venusaur declared proudly.
"Problem with the food? Blame the Doomie couple,"
The strange scene of a Houndoom cuddling with a Starmie both lets out light cries of approval as they stare at Tatsumaki excitedly.
"And lastly, you can blame me and Gust if you didn't want to celebrate your birthday,"
"You two didn't help?" Tatsumaki asks.
"Not one thing," Puff declares.
"Making this plan counts as one," Gust the Pidgeot said.
...
Planet Earth had innumerable changes, but the most noticeable was the tense atmosphere among L&Ms and the unnaturally high levels of energy on it. It's clear that the energy normally spread in the Universe has been concentrated here causing not only an increase in resources.
But the birth of Guardian and Master Pokemons.
If this goes on for another 4 years, Earth's strength might surpass even its heydays. In fact, this isn't a possibility but a certainty.
Project Energy, a project previously run by Eve was to keep Legendaries in one place allowing for more Forbidden Territories to appear, thereby creating more resources and making the world's strength increase.
However, with Eve's now killed or being hunted by Unowns, Arceus decided to take control of this project. Instead, of continuing to gather resentment with other L&Ms by keeping them stuck in one place, Arceus used a combination of Order and Words of God to implant energy-gathering runes at a planetary scale across the world.
6 years later and we have our current Earth, Guardian Pokemons although not common, can be found in almost every territory excluding routes.
Humanity's champions are expected to have a Guardian Pokemon with them, while Elite Fours must have a team of Senior Champions.
Although the world's strength can't be compared with the past, humans of the current time have long since surpassed their past selves.
If not for the lack of time, there may have been more than 2 Master trainers around.
2?
One is Professor Oak himself and the other is the oldest human alive. Having noticed something big would start soon, he grew a powerful (?) team in preparations for what was to come.
...
"Where is he!" Necrozma shouts.
No, she's not searching for Mike, it's far too early to search for him. She's looking for the Progenitor Mew for 6 years straight.
If she knew finding him would be so difficult she would've tried and pulled Arceus into the search. Pity, it's too late now.
Arceus doesn't care half as much if Adam dies or not as her.
His death is great as he seems to be a possible enemy to Arceus. However, even if Adam does kill him she would've to kill Necrozma first.
In that case, his death for the secured safety of the world is worth it, 100% of the time.
Of course, Arceus wouldn't use this method. The world needs 'Arceus' his death brings more harm than good even if Adam could replace him and that's ASSUMING Adam wants such a responsibility.
So for Arceus, whether or not Adam dies is purely on Necrozma. Though it won't change her ending, Necrozma will lose one way or the other.
***
War won't start until Adam dies.
However, Adam is limitless. If he wants to hide who could really find him? Possibly only Arceus using Omniscient can.
Arceus did even prefer if Adam survives, but his unknown intentions in this war are what allowed Arceus to ignore Necrozma arriving on Earth in search of him.
So the first arc would be a mix of multiple perspectives until Adam dies.
Assuming he dies.