Hey Death Cresselia
After you've obtaind the national dex, you can go to the house of a sealer at canaleve city.
There you will find a boy who have nightmares.
His mother will ask you if you wanna go to fullmoon iseland.
There you're gonna see Cresselia, but it will escape.
After that you can find Cresselia all over Sinnoh.
*To locate it you can use the pokeapp MARKING MAP witch you can get in the building of the poketch compeny.
Hope I helped you