Prologue

                                    

                                          

Greninja sprinted through the trees after its commanders. Its webbed feet thumped on the ground heavily, and so did its heartbeat. 
After years of peace in the Kalos Region, something was coming to disrupt the temporary harmony. It was a quiet night that was almost eerie, and Zygarde had picked up on something.
The wind whistled past it as it jumped agilely from tree to tree, trying to keep up with the two Zygarde.
"Up ahead!" Z2 barked. It was the more serious one out of the two Zygarde, with a blue hexagonal core and a glare that demanded respect and discipline. Leading the trio, it was also the fastest and sharpest.
Z1 had a more friendly and relaxed attitude, but it didn't mean it slacked off on its job. Its pink core glowed in the dark as it looked back at Greninja, "Be careful, don't fall behind."
It was easy for them to say: at their 10% Forme, speed was their forte. Skilled at close quarters combat, their canine shaped bodies were capable of running 60 miles per hour. Despite its vigorous training over the years, Greninja was undoubtedly no match for their superhuman speed. 
A purple light glowed menacingly in the distance, followed by a furious screech which vibrated throughout the forest. A powerful wind swept through the leaves, nearly knocking Greninja off its feet.
So that was the monstrosity they were facing off against. 
If it was as the legends described, this was a Pokemon which could surpass both life and death, exceeding the power of both Zygarde combined. Sealed away in the past, it had awakened from its thousand year slumber. They had to stop it no matter what, before it could obtain the full extent of its power. 
The two Zygarde skid to a stop on the scene, and Greninja followed close as well. Looking up in the sky at a menacing indigo orb, a Pokémon's sillohette can be roughly made up. 
"Quick! While it's still weak!" Z2 commanded. The three of them got into formation and went into action, attacking the indigo orb with rounds and rounds of attacks until it shattered into shards, revealing the Pokémon in it. 
The creature was shrouded in darkness, with multiple crooked horns branching out of its forehead and a giant claw like tail.
It was like nothing it had ever seen before. 
Greninja instantly felt a shiver down its spine. The two Zygarde said that whatever they were up against was just a preview of what was yet to come, so Greninja had a precognition that it would be a dangerous opponent. Still, it wasn't expecting a demonic being like this that exerted a pressurizing amount of power. 
With an ear piercing screech, the creature let out a blinding black flash. 
"Stay sharp!" Z2 shouted as it split into two orbs— one magenta and one blue. The two orbs shifted shapes into two familiar creatures that each went off in opposite directions. One resembled a deer while the other, an avian.
"We'll go after Yveltal," Z1 decided, nodding at Greninja. Without a moment of hesitation, it started chasing after the Pokémon that brought upon death while Greninja followed. 
Z2 sprinted into the dark towards the other direction, its pitch black body melding into the shadows. "I'll go after Xerneas then." 
The Pokémon that gave life galloped through the forest as Z2 followed behind in close proximity. Before long, it transformed into a blue orb, disappearing into the night. 
Z2 came to a stop. Xerneas had returned to its slumber at last, which would mean that itscounterpart, Yveltal, had done the same. The threat was gone— for now.