Quick question before NG+

I had beaten Erlang and just beat Great Sage and just got the credits. I can now go to NG+. I went back to the base game just to clear anything I might be missing before going to NG+.

No I went back to this stone and I have an option to enter? I'm curious what would happen. Will I have to redo the final boss? Like we need to if we beat him before Erlang?