If you try to enter Wayward Cave in Pokémon Brilliant Diamond and Shining pearls, you will go in the dark. To go through the maze-like cave you’ll need the TM70, also known as the 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 the Oreburgh Gate, or you can buy it when you arrive in Veilstone City. The Technical Machine Shop (TM) is located on the 3rd floor of Department Store, including the ground floor is the 1st floor.
When you come out of the stairs, you will see two salesmen selling TM. You’re looking for the person on the far wall, facing the camera. The TM will be the last TM on her list, and you can buy it for $1000 Pokémon. Make sure you buy as many as you want to use as each TM will break after using it on a Pokémon.
TM can be taught to many early-game Pokémon such as Turtwig, Shinx, and Budew. Once they’ve learned how to move, you can go to Wayward Cave and open your Pokémon screen from the menu. Scroll down to the Pokémon you taught Flash and select it. There will now be an option to use Flash. Use it to illuminate the cave and find your way to the other side with bright lights.
https://dotesports.com/pokemon/news/how-to-get-the-tm-flash-in-pokemon-brilliant-diamond-and-shining-pearl How to get TM Flash in Pokémon Brilliant Diamond and Shining Pearl