The Lax Incense is one of the items that you can give to a Pokémon to hold. While holding the Lax Incense, the Pokémon is less likely to get hit by an opponent. This is an important item to have if one of your Pokémon gets hit a lot. Here is where you can find the Lax Incense in Pokémon Brilliant Diamond and Shining Pearl.
Before you can get the Lax Incense, you first need to become the champion by defeating the Elite Four. You will also need to complete the Sinnoh Pokédex by seeing all 150 Pokémon in the region. After completing the Sinnoh Pokédex, talk to Professor Oak to obtain the National Pokédex. Go to Snowpoint City and take the boat over to the Fight Area if you haven’t before.
Since you have the National Pokédex, you will be able to travel north of the Fight Area on Route 225. Travel north on Route 225 until you reach a house with two trees to the left of it that can be cut down. On the other side of the trees is an item. Cut down one of the trees and grab the item to get the Lax Incense.