The Forbidden Caverns are a dangerous location where only a trainer who obtained their 8th badge can go, with Sora as company. Pokemon constantly appear in teams of 2 here, much like the dark grass of Unova.
"I'm very curious about the Forbidden Caverns, but you can only go in if you've beaten Sora in a gym battle and she goes with you. I'm no good at battles, but I want to know what secrets are in those caverns.