Pokemon World Disaster

Yveltal

"Yveltal," Ash whispered.
"Who, now?" Greninja asked, staring at the strange-looking Pokemon in front of them. He looked familiar somehow, though.
"Yveltal, the Pokemon of death and destruction. He's usually asleep, but if anyone wakes him up he destroys everything in his path," Ash answered. "We ran into him when we met Diancie, remember?"
Ah, yes, Greninja thought, remembering the diamond-crafting legendary Pokemon. He had only been a Froakie at the time, but he still remembered every moment of Yveltal's rage. The destruction Pokemon had turned half the forest to stone before being stopped by Xerneas.
Yveltal smirked coldly at Ash. "Very intelligent, indeed. I see the doctor boy was quite right when he said you're smarter than you look, Ash Ketchum."
Greninja and Ash narrowed their eyes at the destruction Pokemon. "How do you know my name?" Ash asked coolly. "And what have you done with Brock?"
"I disposed of him," Yveltal replied dismissively, as though Ash had merely asked a question about the weather. "He was in my way. There was no other option."
"No other option? Why you f-" Ash started to yell before a Shadow Beast that Greninja hadn't noticed before suddenly shoved his trainer to the ground. Greninja reached out to Ash to make sure he was all right, but he was already back on his feet and glaring at Yveltal. "What's wrong with you? Why are you doing this?"
"It's quite simple, actually," Yveltal replied, almost pleasantly. "You see, by destroying the entire world, I will forever take my place as the most powerful Pokemon in existence. Not even Arceus himself will be able to rival me in strength. I will be respected as a god, just as I was always meant to be."
"How does what you're doing prove anything except that you're a piece of s-" Ash started to yell again before the Shadow Beast struck him once more. He managed to get back up again, but it was clear that he was winded.
"Stop that!" Greninja shouted furiously.
"Leave him alone!" Pikachu yelled, his cheeks sparking threateningly.
"Language, boy," Yveltal taunted Ash. "Didn't your mother teach you to be respectful of your superiors?"
"I must have missed that particular lesson," Ash gritted out through his teeth. Greninja groaned to himself. Now was not the time for sarcasm.
Yveltal just smiled coldly at Ash. "You have been a great source of entertainment, Ash Ketchum. But I'm afraid that your time is up."
Before Greninja could say or do anything, Yveltal flew towards Ash and grabbed him by the shirt with his talons. The frog-ninja Pokemon watched in horror as Ash's mouth opened in a silent scream of agony, as though Yveltal was preventing him from making any sound. He knew only too well what was happening. Yveltal was trying to kill his trainer from the inside out, just like the Shadow Beasts had attempted mere days ago.
"Ash, no!" Pikachu cried.
The fear little electric mouse's voice snapped Greninja out of his stupor. He hurled a Water Shuriken at Yveltal's talons. It wasn't a super effective attack, but it was enough for the destruction Pokemon to loosen his grip on Ash. Greninja rushed over to check on his trainer and was relieved to see that he was still conscious, albeit barely so.
"Thanks, Greninja," Ash whispered weakly.
"Shh, just relax. You need to rest," Greninja said worriedly. "Infernape!" he called over his shoulder.
"What is it?" Infernape asked, bounding over to stand next to him.
"Can you watch over Ash while Pikachu and I deal with Yveltal?" Greninja asked. Infernape nodded and sat down beside Ash, who was still hovering on the verge of unconsciousness. Greninja nodded in thanks and turned to face Yveltal again, ready to begin the battle of his life.
The destruction Pokemon sighed in disappointment. "You're not running away? But that just makes everything so much more fun," he sighed. He narrowed his eyes threateningly. "Very well, then. Battle stance, Greninja and Pikachu."