The jewel from its chest, which has grown gigantic due to the effects of Mega Evolution, can turn back any attack.
– Pokémon Sun Pokédex
The gem that was on Sableye’s chest has grown giant. According to its Ultra Sun Pokedex, “Bathed in the energy of Mega Evolution, the gemstone on its chest expands, rips through its skin, and falls out.” That’s kind of a graphic description…. I feel bad for it.
Mega Sableye’s ability is Magic Bounce, which reflects status moves back to the opponent. Two of its Pokedex entries say that the gem blocks/reflects any attack, but it does not reflect actual attacking moves.
Stat-wise, Mega Sableye actually has less Speed than Sableye (not that it was fast in the first place, but now it only has 20 base speed); this is reflected in its Ultra Moon Pokedex: “It blocks any and all attacks with its giant-sized gemstone. However, the stone’s a heavy burden, and it limits Mega Sableye’s movements.” In exchange, it gains a lot of Defense and Sp Defense, which, combined with its good Ghost/Dark typing, makes it really bulky. It does gain a bit in Attack and Sp Attack, but neither of those stats are spectacular.