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.
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