Swarms isn't a brand new feature to Diamond and Pearl but it is needed to complete the National Dex without trading. Swarms are used to make some Pokemon more common so you can catch them a lot easier.
In Diamond and Pearl, when you get the National Dex, go to your gender rival's house (Dawn's if you chose to be a boy and Lucas's is you chose the girl) in Sandgem Town and talk to their younger sibling in front of the TV. When you talk to her she'll tell you about a Pokemon and it's location.
Head to where ever she said and search the area. After a few battles you should start seeing the Pokemon around. You can catch it, kill it or whatever.
Everyday the Pokemon will be different. So you can get a lot of different Pokemon in only a few days and the Pokemon they might evolve into can not be caught in-game by another method.
Below are the Pokemon and where to get them.
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