It transforms from its third form to its final form after the player defeats Zinnia in Granite Cave.
At the climax of the Delta Episode, the player feeds it to Rayquaza to give it the necessary energy to Mega Evolve. In Pokémon Omega Ruby and Alpha Sapphire, no Rayquaza can Mega Evolve until the Rayquaza in the story has consumed the Meteorite (there is no similar requirement in Generation VII).
The player can obtain the Meteorite by defeating Mega Rayquaza in Stage 300 at the end of Roseus Center. The Meteorite was also available as a prize during the Shiny Mega Rayquaza Competitive Stage that ran on both the 3DS and Mobile versions from February 21 to 28, 2017.
The Meteorite enables the following Mega Evolutions:
The Meteorite appeared in Fight for the Meteorite!. Professor Cozmo searched for it on Mt. Chimney and succeeded in finding it with the help of a scanner. As soon as he had found it, however, Team Magma arrived and attempted to steal it, but Team Aqua's arrival gave Professor Cozmo and Ash a chance to get away with the Meteorite. While hiding from Team Magma, Professor Cozmo explained to Ash that the Meteorite was a part of a large asteroid that had fallen from space.
The Meteorite was soon stolen by a Golbat belonging to Team Magma and placed inside of a machine meant to make Mt. Chimney erupt. As there was no time to get the Meteorite out of the machine in time to prevent the eruption, Professor Cozmo instead chose to push the machine into the volcano, destroying both it and the Meteorite.
In this manga, the Meteorite is known as the Grand Meteor (Japanese: グランメテオGrand Meteor). It is a stone from space that has the ability to stop natural energy. It was originally discovered by Professor Cozmo.
In Bubble Bubble Toil and Azumarill II, Team Aqua tricked Professor Cozmo into letting them use the Grand Meteor to stop the volcanic activity of Mt. Chimney, which would allow Hoenn's sea level to rise, aiding in their schemes. Though Sapphire fought to stop them, she was unable to prevent the machine powered by the Grand Meteor from making Mt. Chimney dormant. After Team Aqua escaped, Sapphire tried pursuing them, but was only able to find a broken-off Grand Meteor fragment.
In Omega Alpha Adventure 20, Professor Cozmo discovered that the meteoroid threatening to destroy the world was actually the Grand Meteor, which had somehow returned to space and increased in mass. Not wanting to make things confusing by calling them the same name, Nonkichi suggested naming the meteoroid Grand Meteor Delta.