Ghost-type trainers in Pokémon Brilliant Diamond and Shining Pearl should head to Celestic Town during their playthrough as its close path provides a powerful MT30 Shadow Ball move. Here's how to find it.
This powerful dark move in Pokémon Brilliant Diamond and Brilliant Pearl is found on Route 210. You can enter it after receiving a unique potion from Cynthia and clearing the Psyduck barricade blocking your path. You'll also need the hidden Defog move to proceed through this otherwise hidden landscape. You can look for him in the Great Marsh in Pastoria City after beating the Veilstone City Gym Leader. Route 210 is east of Celestic Town and northwest of Solaceon Town.
Once you are on Route 210 from the east side, pass the first piece of grass and go up the stairs. Past the first wooden bridge, you will see a thinner intersection that you cannot walk on. In a dangerous move from your protagonist, ride your bike and then follow the passage left and down. You will see boulders in your way. Use the hidden Rock Smash move to get through them. The TM30 Shadow Ball will be waiting for you on the other side.
Shadow Ball's power and accuracy are very effective at speeds of 80 and 100 respectively. It will also reduce the target's special defense stat, which would be perfect for a Pokémon like Alakazam or Gengar.