Of all the new Mega-Evolved Pokémon, Mega Diancie is probably the trickiest to obtain. Originally revealed before Omega Ruby and Alpha Sapphire were released, the only way to get Mega Diancie is to trade a Diancie obtained from Pokémon X/Y.
At the time of writing, Diancie was only (legitimately) available as a limited-time serial code distribution during late 2014 and the serial code can no longer be obtained. So if you missed Diancie, you’ll have to cross your fingers that Nintendo re-distributes it later or ask a friend, etc.
With Diancie in your party, the next time you visit a Pokémon Centre, a bizarre duo will interrupt you and attempt to sweet-talk you into giving away your Diancie. No matter what option you choose, you will keep your Diancie and be given its Mega Stone, Diancite .
Note : Since the Diancite is new to Omega Ruby and Alpha Sapphire, it cannot be traded back to X/Y. However, you can of course trade Diancie herself back.