What If Ash was in Pokemon Horizons?

Stellar Finale

The trainers oversee the Laquium Core, before Hamber suddenly arrives. He and Diana share a nod before attack the Core with their Pokemon. Coral, Sidian and Chalce join in, causing the remaining trainers to gain a determined glint in their eyes. Roy looks up to the Black Rayquaza, "Let's keep Lucius' promise to protecy the world's future!"

Lucius' spirit suddenly appears and places his hand on Roy's shoulder. Causing Roy's Key Stone to glow and the Black Rayquaza transforms into Mega Rayquaza. Amethio looks to the White Zygarde. "Grandfather wanted me to follow my own path, but I can't do that if the world isn't saved from Laquium."

Gibeon's spirit appears and placs his hand on Amethio's shoulder. Mewtwo suddenly disappears and reappears with Squihy in his arms. "The only way for a Zygarde to go beyond its Complete Forme, is to introduce a third Core."

Squishy merges with the White Zygarde as Z2 once did, Amethio's Key Stone glows as the White Zygarde transforms into Mega Zygarde. The two Dragon types prepare to attack and the Laquium Core senses that, sprouting buds out of the ground. Ash pulls out Melmetal's Poke Ball and it enlarges thanks to his Dynamax Band. Mewtwo tosses Ash a Mega Stone, "You'll need more than Gigantamax to assist them."

Ash sees the spirit of Sir Aaron standing across from them, the two of them smirk at each other and share a nod. "I can't give up, I won't. Not until every Pokemon in the world knows that I just wanna be their friend!"

Ash tosses Melmetal's Poke Ball and reveals Gigantamax Melmetal to everyone, he then taps his Key Stone to Mega Evolve Mewtwo into a different form than when it was under Giovanni's ownership, which Ash guesses is Mega Mewtwo Y.

Ash pointed towards the buds, "Clear Rayquaza and Zygarde a path."

Melmetal strikes the ground with its fist, creating spikes of G-Max Meltdown to destroy the buds that Mewtwo boosts its power and quantity with Psychic.

"I'm gonna do all I can to help out too." Silver declared, "I wont let my father's influence ruin the world!" Silver pulls out a golden Poke Ball with a "GS" logo, a Poke Ball Ash recognizes.

"The GS Ball?" Ash's eyes widened, "How'd you get-

The GS Ball finally opens, revealing a Lugia with pink acsents instead of blue, which Ash stares at in awe. "Use Nihil Light!" Amethio commanded.

The White Zygarde powers through the Core's defenses, allowing an opening for everyone else. "Aeroblast!" Silver commanded.

"Dragon Acsent!" Roy commanded before turning to Skeledirge, "Help out with Torch Song."

Skeledirge uses Torch Song as the Black Rayquaza flies through the move, absorbing the attack to add more power to Dragon Acsent. It and Silver's Lugia managing to damage the Core even further. Mewtwo and Melmetal strike with Shadow Ball and G-Max Meltdown as a finishing blow. The Laquium Core exudes a cloud of Laquium mist as a last resort, Pikachu and Meowscarada easily resist the effects, encouraging the other Pokemon to resist the effects as well. Liko feels a presense above her and sees Rystal's spirit smiling down at her. Liko smiles, "Pagogo, finish this!"

Pagogo transforms into its Stellar Form and Terastallizes each Tera-type representative, Terastallizing Pikachu and Ash's Muk for extra power. Pagogo engulfs the Laquium Core in a barrier and rises to the sky as the Pokemon float to the ground and the Terastallization comes undone. As does the Mega Evolutions and Gigantamax as the clusters of Laquium supporting the Core begin to shirk and cease to exist.

"It's finally over..." Ash sighed in relief.

"No, no, NO!" A voice shouted, Spinel climbing himself out of the Laquium rubble. Umbreon eagerly rushes over to him to see if he's okay. "I cannot allow this," he picks up a small Laquium crystal. "Umbreon, take this."