Congrats! You made it out somehow!
And look at that, the prize was waiting for you out here all along! Surprised you missed that going in actually...
You've found:
648 NP!
Baelia Stamp
First time finding anything! Woohoo! Now there are only 100 plot related items I still need (including a few discontinued, plus a few map pieces to complete a set).
Edit 9/1:
Chapter 10 is out! I found the final extra note for the achievement. New BD challenger and volunteer shift. Looks like we should be getting a big battle soon based on the achievements list. I should probably stop procrastinating on finalizing my set. I need to go look at the list again.
I now need 98 items (but that's including most of the new BD prizes I should get in the next few days). It's going down every few days. I've still only gotten one prize from the caves. Maybe tomorrow?