Things You'll Need
Instructions
Obtain an event "Arceus" (chances are, you'll have to trade for one, as the giveaway ended some time ago). Put Arceus at the top of your party and go to the Ruins of Alph.
Try to enter the house in the Ruins of Alph, which will result in a man speaking to you about Arceus and leading you to Mystri Stage in the Sinjoh Ruins. Go outside and talk to the hiker, who will lead you to a house with a PC and an Abra. Deposit at least one of your Pokemon (except for Arceus) into the PC.
Speak to Cynthia, who will lead you to the ruins again. Watch the event and then step on the sphere on the bottom-left to obtain a level-1 Dialga holding an Adamant Orb; you will then be teleported back to the Ruins of Alph.