After doing so, you'll receive a Castform from the professor of the Weather Institute. Make sure you have room in your party, as you can't get it if your party is full.;
, You get the Eon Flute after completing the Primal Groudon/Kyogre storyline at Sootopolis City.
 This is required for Tornadus/Thundurus to appear.
, Go to the thundercloud and say yes to the question you're given to go into battle with Tornadus in Omega Ruby and Thundurus in Alpha Sapphire.
Defeat Team Magma/Aqua at the Weather Institute at Route 119.
Get the Eon Flute.
Place the Castform you received into your party.
Use the Eon Flute to summon Latios/Latias and soar into the sky.

,
Soar to Fortree City, where you'll find a thundercloud hovering above.
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