From serebii:
"Further details have been revealed about the Pokémon, Mega Sableye. It is confirmed that Mega Sableye retains its Dark/Ghost-type, but rather than keep any of the abilities, it gets the ability Magic Bounce. It has increased Defense and Special Attack stats, but decreased Speed. During battle, Mega Sableye shelters itself behind the jewel, from where it can take advantage of any openings its target leaves exposed."
Tank much