Announced a good while ago, a particularly tempting distribution has finally gone live in Pokémon Omega Ruby & Alpha Sapphire. On this occasion everyone with one of these games can grab a Mythical Diancie, but only until 27th July; importantly, this is a 'mon that can't be found in the wild.
It's the usual drill to get hold of one, as you go to Mystery Gift, then Receive Gift and then Get Via Internet. Its moves are Diamond Storm, Moonblast, Dazzling Gleam and Protect, while if you're still lacking the Diancite Mega Stone you can rock up to a Pokémon Center in-game to get one.
Be sure to grab one while you can!