Part 8: Beat Wallace — Sootopolis City Gym, badge 8

Beat Wallace — Sootopolis City Gym, badge 8 opens 1 area.

Where to go

1. New Mauville

   Wild: Voltorb 65% · Lv 12, Magnemite 35% · Lv 12

Everything you can have caught by this checkpoint · Open the interactive guide

It also doesn't say when 9 of its areas open up, so they are listed together in the last part rather than guessed into the story.