The official Pokémon Omega Ruby and Alpha Sapphire websites have updated to reveal the details for Mega Sableye. It has the Ability Magic Bounce and keeps its Dark/Ghost type.

Picture	Info
	Species: Darkness Pokémon
	Type: Dark/Ghost
	Ability: Magic Bounce
	Height: 1'08"
	Weight: 354.9 lbs.

Mega Sableye

Sableye has concentrated all of the energy from its Mega Evolution into the red jewel on its chest, which increases in size and detaches from its body. This enlarged jewel boasts incredible hardness, and it is said that no attack can even make a scratch on it!

When Sableye Mega Evolves, its Defense and Sp. Attack stats both get big boosts. Its Speed stat does decrease slightly, perhaps because of the weight of its huge jewel. Thanks to its Magic Bounce Ability, any moves that affect only stats or status conditions will be bounced back at its attacker.

Sources: http://www.pokemon.co.jp/ex/oras/pokemon/18.html and http://www.pokemonrubysapphire.com/en-us/pokemon/mega-sableye