Where is cressilia?

Cresselia is a legendary Pokémon, not a real location. It is a Psychic-type Pokémon from the Generation IV games, Diamond and Pearl.

While it's not a real place, you can find Cresselia in various Pokémon games:

* Diamond, Pearl, and Platinum: On Fullmoon Island.

* Black 2 and White 2: In the Dream World.

* Pokémon GO: As a Legendary Raid Boss.

* Pokémon Brilliant Diamond and Shining Pearl: In the Grand Underground.

* Pokémon Legends: Arceus: In the Hisui region (as a post-game encounter).

It's important to remember that Cresselia is a fictional creature, not a real place.