The Regis are back!
This will only happen if you have the special event Regigigas. Have it in your party and go to either the desert north of the Resort Area, Mt. Coronet, or Iron Island. There should be a cave somewhere deep in each of those locations. If the Regigigas is in your party, there should be a puzzle to solve in each of them complete it for a chance to battle Regirock, Regice, or Registeel. They are all at Lv. 30.