The Battle Mountains are a battle facility meant for very strong trainers. It is located near the end of the first demo. There are many things a player can do here.
TM Shop:
A shop to buy TMs, either for BP or Money. The shopkeeper, Shiny regrets using all the TMs in existence and likes money. Don't talk to him too much or this clip will replace the background music everywhere permanently until further notice. https://www.youtube.com/watch?v=ys9Q_VHNMZM
Battle Modes:
Saberz's Home:
The founder of the Battle Mountains, Saberz lives here. However, you won't see him here until you beat him in the Mountain Chain. Afterwards, he will give a different set TM everyday. Once you have beaten Saberz on Super Single/Double on Mountain Chain, Saberz will offer you his Shiny Squirtle for any Pokémon. It knows the moves Aura Sphere, Water Pulse, Dragon Pulse, and Water Spout. It is also holding the Blastoisinite.
Secret Bases:
In the Battle Mountains, there is a small unexplored cave where the player can create their own secret base. They can upload it online via codes. There are many characters to add in the base, such as move tutors or Blissey users. You can even decorate it.