Complete the main story and complete your pokedex then head to Canalave City. Go inside the house that is near to the sailor that takes you to Iron Island. There will be a boy who's stuck in a Nightmare then talk to his mom.Go outside and talk to the sailor and he will ask you a favor to go to the crescent moon island and he'll take you there.
Chat with our AI personalities
when the boy is sleeping in canclave talk to him then talk to the girl thentalk to the sailor he will take u to crescent or fulmoon island
You have to get the national pokedex then go to the guy who brings you to iron island and he'll ask you to help wake up his kid by going to crescent moon island. You will find Crescellia there and he will do the same as mespirit.
on crescent moon island in canalave city after you beat the game
New moon island is only avalible in Pokemon pearl, diamond and platinum
you can get Cresselia at full moon island Darkrie at new moon island diagla at spear pillar for diamond and palkia at spear pillar for pearl