For the first time ever in a generation, Pokémon Omega Ruby & Alpha Sapphire have introduced new abilities into the game that didn't exist in the previous game of the generation

Primordial Sea

US Name: Primordial Sea
Jp. Name: Sea of Beginnings (はじまりのうみ)

Effect:
This ability will create strong rain. This prevents other weathers from being activated, stops Fire-type moves working and lasts until Primal Kyogre exits battle.

Pokémon:
Primal Kyogre

Desolate Land

US Name: Desolate Land
Jp. Name: Land of Endings (おわりのだいち)

Effect:
This ability will create strong sunlight. This prevents other weathers from being activated, stops Water-type moves working and lasts until Primal Groudon exits battle.

Pokémon:
Primal Groudon

Delta Stream

US Name: Delta Stream
Jp. Name: Delta Stream (デルタストリーム)

Effect:
This ability will create a new weather called Strong Winds. This prevents other weathers being activated, and over-rides all weathers including Desolate Land and Primordial Sea. It also removes the effectiveness from moves that would be Super Effective on Flying-type Pokémon. It lasts until Mega Rayquaza exits battle.

Pokémon:
Mega Rayquaza