If you try to enter Wayward Cave in Pokémon Brilliant Diamond and Shining Pearl, you’ll be walking in the dark. To get through the maze-like cavern, you’ll need TM70, also known as Flash, Without it, you’ll be walking around in the dark and won’t be able to see much on your screen.
Getting Flash is a simple process. You can find it in the basement of Oreburgh Gate, or you can purchase it once you get to Veilstone City. The shop that sells the Technical Machine (TM) is on the third floor of the Department Store, counting the ground floor as the first.
Once you exit the stairs, you’ll see two salespeople selling TMs. You’re looking for the one on the far wall, facing the camera. The TM will be the last one on her list, and you can buy it for 1000 Pokémon Dollars. Make sure you buy as many as you plan on using because each TM breaks after using it on a Pokémon.
The TM can be taught to many early-game Pokémon such as Turtwig, Shinx, and Budew. Once they have learned the move, you can head to Wayward Cave and open up your Pokémon screen from the menu. Scroll down to the Pokémon you taught Flash to and select it. There will now be an option to use Flash. Use it to illuminate the cave and make your way to the other side with a bright light.