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The ability of Mega Diancie in Pokémon Omega Ruby and Alpha Sapphire is Magic Bounce.
The new Mega Evolutions that premiere in Pokémon Omega Ruby and Pokémon Alpha Sapphire are Swampert, Sceptile, Latias, Latios, Rayquaza, Diancie, Beedrill, Pidgeot, Slowbro, Steelix, Sableye, Mawile, Sharpedo, Camerupt, Altaria, Glalie, Salamence, Metagross, Lopunny, Gallade and Audino which officially adds up to a total of 46 species being capable of Mega Evolution and there are 48 Mega Evolutions overall as of Pokémon Omega Ruby and Alpha Sapphire's release.
Seviper does not have a Mega Evolution.
Mega Swampert has the ability Swift Swim, which doubles its Speed stat in Rain.
Yes, Steelix can mega evolve in Pokemon Omega Ruby and Alpha Sapphire. If you are playing the Japanese Pokemon Omega Ruby and Alpha Sapphire special demo, you can receive a free Steelix with a Steelixite to send to the full games. By normal methods, you can get a Steelix by trading for it from another player who owns a Onix holding a Metal Coat, then get the Steelixite at the bottom floor of Granite Cave next to a hiker. It requires a Mach Bike to reach it.
Mega Sceptile has the ability Lightningrod, which makes it immune to Electric-type attacks and raises its Special Attack by one stage when it gets hit by one.
Swablu evolves into Altaria at level 35. Altaria can mega evolve in Pokemon Omega Ruby and Alpha Sapphire when it is holding a Altarianite during a battle.
After you complete the Petalburg Gym, you can surf on route 118. When heading east, you will meet up with Steven and he will transfer you to the Southern Island. After completing the events that occur, Steven will give you the Mega Bracelet
Mega Sableye has the ability Magic Bounce, which "bounces" back all targeted status attacks back to the user. For example, if an opponent uses the move Thunder Wave on your Mega Sableye, Thunder Wave would affect the opponent instead of Mega Sableye.
Mewtwo cannot evolve, but it can mega evolve temporarily in Pokemon X, Y, Alpha Sapphire, and Omega Ruby by letting it hold a Mewtwonite X or Mewtwonite Y.
These are the Pokemon who can mega evolve in X, Y, Omega Ruby, and Alpha Sapphire: Venusaur, Charizard, Blastoise, Alakazam, Gengar, Kangaskhan, Pinsir, Gyarados, Aerodactyl, Mewtwo, Ampharos, Scizor, Heracross, Houndoom, Tyranitar, Blaziken, Gardevoir, Mawile, Aggron, Medicham, Manectric, Banette, Absol, Garchomp, Lucario, and Abombasnow. Only in Pokemon Omega Ruby and Alpha Sapphire: Beedrill, Pidgeot, Slowbro, Steelix, Sceptile, Swampert, Sableye Sharpedo, Camerupt, Altaria, Glalie, Salamence, Metagross, Latias, Latios, Rayquaza, Lopunny, Gallade, Audino, and Diancie. These Pokemon are considered Primal and do not fall into the Mega Evolution category completely: Kyogre and Groudon.
Sharpedo does not evolve any further, but you can mega evolve it temporarily in Alpha Sapphire and Omega Ruby by letting it hold a Sharpedite.