Like many games, Pokémon Omega Ruby & Alpha Sapphire have got a variety of events that occur when you bring a specific Pokémon you received in an event to a place. Some of these are just text while others actually give you things
Location: Rustboro City, Route 114, Mossdeep City
The Shiny Beldum distribution was live for the first two months of the release of Pokémon Omega Ruby & Alpha Sapphire. If you take this Beldum, or its evolved form, to Mr. Stone in Rustboro City, he will be shocked and begin saying how it must be one of a kind and how sublime it is. He ends up saying that he wonders how it'd stand up to Steven's own Beldum
In addition to this, Steven Stone's letter about the Beldum you get given in the game also now mentions a second, special, Beldum that he sent you.
If you speak to Steven in Mossdeep City with it in your team during the Delta Episode, while he is in the Mossdeep Space Center, he will reference how it reminds him of a time he faced off against Rayquaza, teamed with a young man and his black Charizard, referencing the Mega Evolution anime special.
Finally, in Route 114, if you show it to the Fossil Maniac, he will mention how Steven travelled as far as Kalos in a hunt for this special Beldum.
Location: Any Pokémon Center
Once you have traded/transferred Diancie over to these games from X & Y, if you have it in your team then, when you next go to leave a Pokémon Center, you will be interrupted by a Gentleman and a Black Belt, who are working for a child. Like in X & Y, they will try to convince you to hand over your Diancie to them. Regardless of what you say, the child will come in and tell them he is no longer interested in Diancie. He then gives you the special Mega Stone, Diancite, which is required for Diancie to Mega Evolve
Location: Any PokéMart
Once you have obtained a Hoopa from an event, if you have it in your team , when you go to a PokéMart you will see an extra clerk. Speak to this clerk and he will tell you a story of how a Pokémon' was so powerful and so evil it was confined in a special bottle. He will then give you the bottle. This bottle is the Prison Bottle and allows for you to change Hoopa's form into Hoopa Unbound