If you used an action replay to do the Arcues event, then there will be things unlocked.
Before, make sure you have Arcues In your Party
Go to Orebrugh City and go to the Mine. There will be this hiker dude.
He will talk about somthing about how plates are related to sinnoh's myth.
He will give you a flame plate, talk some more, and leave. you can find him again on the second floor of the canalve city library. Then he will talk about sinnoh's myth, then Arcues.
then he will say he wrote books that you can read. on the second floor wear the tall book case with blue books are.
Another one is when if you have Arcues in your party, go to celestic town and go to the cave with the pictures of Mesperit,Uxie,and Azlef. Press A on the wall then cynthia will come. she will say that something she will talk for a while about stuff about sinnoh's myth and how arcues was born in an egg. she after she will apologize how long she talked.
there are maybe more things but those are just the ones i know.