In Pokemon Brilliant Diamond and Pokemon Shining Pearl, players can give Pokemon items to hold. Some items can't be used by Pokemon, but others are just there to help boost a Pokemon's stats or the effectiveness of their Moves. There are even Pokemon-specific items (such as the Light Ball), which are items with effects that can only be brought out by a certain species of Pokemon. Luckily, items such as the Expert Belt can be used by any Pokemon.
The Expert Belt slightly increases the damage of super-effective moves. Meaning if a Pokemon were to use the Fire-type move Ember on a Grass-type Pokemon, the damage of that Ember will be stronger than it would have been without the Expert Belt. Unlike the Focus Sash, the Expert Belt won't break after being used. To get the Expert Belt, players will need to make some progress in Brilliant Diamond and Shining Pearl's story.
Players will need the HM Surf to get the Expert Belt. They can get Surf by helping Cynthia's grandmother in Celestic Town. To use Surf outside of battle, they'll need to get the fifth gym badge from Fantina in Hearthome City.
With these in hand, use HM Fly to get to Sandgem Town. Go south and use Surf to get onto Route 220. The trainers in this area will have Pokemon at level 28-31, but they shouldn't be too much of a problem. Defeat them or go around them, and continue to the right into Route 221.
Expert Belt Location in BDSP
Route 221 has Ramanas Park, which is where players can catch Legendary Pokemon in the postgame. However, this isn't where the Expert Belt is. The Expert Belt is in the house to the north of Route 221. So, go right past the fisherman and there should be a path leading north among some trees. Keep heading north and there will be a house with a blue roof. Go inside.
While players can get the Expert Belt here, it's not guaranteed. The old man will say a random number each day, and this number can range from 1 to 100. These numbers indicate Pokemon levels. Meaning if he's asking for number 48, he'll want to see a Pokemon from the player that is exactly that level. The easiest method is to catch a Pokemon at the level he wants (if it's possible), or just level up one of their own and show it to him. The number refreshes every 24 hours. So if the level is too high and players want to try for something lower, they can come back the next day.
Unfortunately, the reward is randomized. Players can get either the Focus Sash, Expert Belt, or the Black Belt and will need to do this task two more times to get the remaining items. Since Expert Belts can't be bought at the store or found on Wild Pokemon, this is the only method of obtaining one in Brilliant Diamond and Shining Pearl.
Pokemon Brilliant Diamond and Pokemon Shining Pearl are available now on Nintendo Switch.