The Shiny Gengar distribution for Pokémon X and Pokémon Y is underway in North America. From today until October 26th, you’ll be able to pick up a Shiny Gengar via GameStop stores. The Gengar will come holding a Mega Stone that is capable of Mega Evolving him.
The Shiny Gengar will come with the move Sludge Wave, which Gengar cannot normally learn. Upon Mega-Evolving, he will gain the ability Shadow Tag, which prevents foes from fleeing or switching out in battle.
As mentioned in a past report, once the Shiny Gengar event concludes, from October 27th onward, GameStop stores will begin to distribute a Diancie, and transferring this Pokémon to the upcoming Omega Ruby and Alpha Sapphire games will allow you to Mega Evolve it. More details on Mega Diancie can be found here.