Defog is one of the TMs you can find throughout Pokemon Brilliant Diamond and Shining Pearl. This TM is also one of the hidden machines that the Poketch can use to remove obstacles in your path while you traverse the map. Specifically, Defog will remove fog in areas with dense fog. The is great for areas like Route 210. Here is where you can find the TM.
Before you can get the TM, you will first need to progress pretty far in the game. During your journey, you will come across Veilstone City. After defeating the gym leader here, you will be directed to go to Pastoria City. Take the route south of Veilstone City and be sure to rest up before leaving since there are plenty of trainers to fight along the way.
Once you reach Pastoria City, head up to the Great Marsh. The building is in the northern section of the city and it will cost you 500 dollars to get in. After entering the Great Marsh, head to the right to find an NPC with green hair. Talk to them and they will gift the TM Defog.
