"Unlocking Rayquaza's Mega Evolution in Pokémon Go: Mastering Meteorites"
TL;DR
Pokémon Go players can Mega Evolve Rayquaza by teaching it the move Dragon Ascent using the rare Meteorite item. Meteorites can be obtained through special research, Mega Rayquaza raids, or by attending in-person or global Go Fest events. Mega Rayquaza's Mega Evolution provides temporary boosts to CP, power, and various perks, and requires 400 Mega Energy to evolve.
Want the full story? Read the original reporting
Read on Polygon