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The trio comprising of Groudon, Kyogre and Rayquaza is known as weather trio. They are named because of their powers to control weather. The Groudon's power is associated with land. Kyogre has it's control over the seas and oceans. Rayquaza is considered as trio master of the two and it has mystical powers to control skies and space. All of them are very hostile and aggressive. Let's discuss each of them in detail.
Rayquaza serves as the master of this trio. Whenever other two member of trio engage in a battle with each other, it stops them from battling. It has a signature ability of Air Lock which shows how it manages to rule over sky. It got a fighting nature too. It is more powerful than Groudon and Kyogre and has base stat of 680. It divides the land, ocean and the sky.
Kyogre is a water type member of this trio and it is rival to Groudon. They battle between them is always severe. Both try to attack each other in furious manner. After battling, it goes to depth of seas and sleeps there for long time in order to heal itself. It has ability to expand oceans, opposite to Groudon who can expand the lands.
Groudon is the third member and holds the continents. It has a old rival Kyogre. Although Kyogre has a type advantage against it but still it holds a very good fight. Groudon can expand the lands and after the fight he sleeps in magma. Whenever it wakes up, the volcanoes erupt. It's body resembles the shape of earth's crust.
On boxarts of OR/AS, it is seen that they are not normal Groudon or Kyogre. They receive a new form. Some fans speculate it their Mega form while others call it alternate forms; Omega Groudon and Alpha Kyogre. Their bodies are shown be be enlarged and lightened. Lets wait and see if these are new forms or not.
Groudon contre Kyogre by Groudon