Cresselia is a wandering Legendary Pokémon that you can eventually spot in Pokémon Brilliant Diamond and Brilliant Pearl. It is a Pokémon that takes some time to find, and being a legendary wanderer, it can be difficult to track down. Here's what you need to know about how to discover Cresselia in Pokémon Brilliant Diamond and Brilliant Pearl.
It is important to note that you will not find Cresselia until you reach the end of the game and unlock the national PokéDex. This happens when you get to the end of the game and have encountered all the Pokémon in the Sinnoh region. Once you've done that and received the National PokéDex, you'll be ready to meet Cresselia.
Your next step is to go to Canalave City. There, you will have to talk to a sick child and their mother. After that, you will be ready to go to Fullmoon Island. You can do this by talking to the sailor in town and asking to visit the island. When you visit the island, you will meet Cresselia, who will start wandering around the map. You'll need to track and find it using your Pokétch's Marking Map application to make it easier. Stray Legendary Pokémon bounce around the entire map, so tracking him down will be challenging, well worth it if you're trying to catch him.
We recommend that you save your game just before each encounter to make sure you have the best chance of catching this Pokémon and adding it to your collection.