Nintendo is still rolling out Mega Stones in various ways, and this one just happens to be easy peasy code-based.
If you own Pokemon Sun or Moon, log in, then type in the code "AZUL." You'll get a set of Mega Pidgeot, Mega Steelix, Mega Heracross and Mega Houndoom stones for your trouble. In a sort of balancing measure you can't use the stones online until May 16, to ensure that everyone is able to grab them and is on equal footing, which makes perfect sense.
I've been winding down my Pokemon Sun and Moon time since I nabbed every 'Mon, but it's still one of my favorites in the series. It has so much character -- I really hope we don't have to wait too long for a followup or even a third version.
Pokemon Sun and Moon [Serebii.net]