The Lagging Tail is one of the items you can make a Pokémon hold. Doing so will cause the Pokémon holding the item to move much slower than normal, resulting in them attacking second in battle more often. This item is great if you are a fan of the move Trick Room. Here is where you can find the Lagging Tail in Pokémon Brilliant Diamond and Shining Pearl.
Before you can get your hands on the Lagging Tail, you will first need to complete two objectives. First, you will need to defeat the Elite Four and become the champion. Second, you will need to complete the Sinnoh Pokédex. This is done by seeing all 150 Sinnoh region Pokémon. Once that is complete, head over to Snowpoint city and take the boat over to the Fight Area.
Once you have the National Pokédex, you will be allowed to access Route 225. Head north on the route until you reach the Survival Area. You will know when you’re in the right spot thanks to the Pokécenter and PokéMart in the area. Take a rest if you need, there will be a few battles ahead.
Head east of the survival area and use Rock Climb on the rocks slightly southeast of the Survival Area. This will lead to a cliff with an Elite Trainer. Use Rock Climb on the rocks to the right of the ones you used to get on the cliff. Go through the tall grass and use Rock Climb on the rocks that lead back up. At the top of the rocks, you will see an item. Pick it up to get the Lagging Tail.