The Pokémon mass outbreak/swarm mechanic was first introduced in Pokémon’s second Generation and appeared in every Pokémon game up until Black 2 and White 2. The feature was abandoned starting with Pokémon X and Y, but since Pokémon Brilliant Diamond and Shining Pearl are remakes of the Generation IV games of the same names, the swarm mechanic is back once more. Here’s how to unlock this mechanic in Brilliant Diamond and Shining Pearl, as well as how it works.
First, you have to defeat the Elite Four and unlock the National Pokédex. Once that’s done, head over to Sandgem Town and enter Lucas/Dawn’s house, which will be right under the town’s Pokémon Center. Speak to Lucas/Dawn’s sister, and she’ll let you know of a swarm of Pokémon sighted on a particular route. Venture to the indicated route, and you should see 3D models of the Pokémon popping in and out of the tall grass. Just wander in the grass a little to encounter a member of the swarming Pokémon species.
Both the swarm’s species and location change daily, and you can check each day’s swarm by either talking with Lucas/Dawn’s sister or consulting the text box that appears when pausing the game. As swarms only unlock after acquiring the National Pokédex, many of the Pokémon encountered will be part of it, making regular paying attention to swarms an effective way to fill out your Pokédex.
It’s also worth noting that if you and your friends are all in the same group, every member of the group’s swarms will be the same on any given day. This can be especially useful if a particular Pokémon you’ve been gunning for is part of a swarm on a day that you’re not able to play; your groupmate could catch an extra one and trade it to you later.