Mega Rayquaza was recently announced for the upcoming Pokémon Alpha Sapphire and Pokémon Omega Ruby games. Upon mega evolving, Rayquaza gets a boost in most of its stats, including Attack and Special Attack. Mega Rayquaza also receives the exclusive "Delta Stream" ability. This ability prevents Drizzle, Drought, Sand Stream, and Snow Warning, Rain Dance, Sunny Day, Sandstorm, and Hail from activating, and replaces any existing weather effects with its own "Strong Winds" weather condition.
The announcement states that " a very limited number of Pokémon" may be able to change the Strong Winds weather condition, which could imply that Primal Groudon and Primal Kyogre are still able to activate their abilities of Desolate Land and Primordial Sea, since these are the only two weather changing abilities that are not specifically mentioned as being prevented.
Strong Winds weather condition only lasts as long as Mega Rayquaza is out and goes away once Rayquaza faints or is switched out. This is much like Desolate Land and Primordial Sea abilities. Strong Winds makes moves that Flying types are normally weak to deal normal damage, meaning Mega Rayquaza will no longer be weak to Rock type moves. It is unclear how Strong Winds will interact with secondary types. For example if it makes Ice moves deal only normal damage to Rayquaza, or if the ability will only remove its flying weakness to Ice types but leaving its Dragon weakness to Ice type; making it receive double damage from Ice moves instead of quadruple.
Rayquaza is also receiving the new "Dragon Ascent" move. Contrary to its name, it is a Flying, not a Dragon typed move. It is described as "one of the greatest power ratings of all Flying-type moves" however an exact power is not specified. Like Close Combat, which has 120 power, Dragon Ascent lowers the user's Defense stat and Special Defense stat after use.