New information on Sableye’s Mega Evolution has been revealed on the official Pokémon Omega Ruby and Alpha sapphire website!
Mega Sableye will have higher Defense and Special Attack stats at the cost of its Speed being slightly reduced. It also has the ability Magic Bounce, so any non-damaging status moves, stat reducing attacks, or entry hazards will be reflected back at the user.
During battle, Mega Sableye shelters itself behind the jewel, from where it can take advantage of any openings its target leaves exposed. — Official Site