Hitch a ride with the sailor in Canalave City and you'll be sent to Fullmoon Island (or something along those terms). You'll see Cresselia. It'll flee when it see's you, and leave the lunar wing.
After you gain the national dex (Beat the PKMN league champion Cynthia, see all the pokemon, then go to Prof. Rowan's lab and Prof. Oak will give you it) Go to Canalave city and enter the Sailor's house. The small boy will be lying in bed. If you talk to his mother, she will say that he's suffering from nonending nightmares and is unable to wake up. Speak to the sailor at the docks and he will ask a favour of you. He will then take you tpo a place called Fullmoon island. When you get to the forest, there will be a crescent moon-shaped puddle and in the center, Crescellia will be waiting. Talk to it, It will be recorded in your pokedex, and will fly off, leaving the item you need - The lunar wing - where it was. Get the lunar wing and go back to the sailor's house. Giove the boy the lunar wing and he will wake up. You can now track down Crescellia on the marking map poketch app* and capture it.
*When you get a certain amount of gym badges, go to the poketch company in Jubilife city and you'll get the app.
Once you have the Lunar Wing from Full Moon Island, (A place to catch Darkrai (event only) and to see Cresselia) head back to Canalave City and into the Sailors House. Talk to the boy in the bed and give it to him. This saves him from a horrible dreamcaused by the presence of Darkrai, but you can only fight Darkrai through and event.
give it to the kid who's sick in the sailor's house
go to canalave city talk to the sailor he'll take you to fullmoon island and there you'll meet cresselia and she'll flee and leave the lunar wing behind
It is Cresseliayou meet Cresselia after the hall of fame in pokemon black+white 2 you need to retrieve the lunar wing first from the Strange House near Reversal Mountain. then go to marvelous bridge and you will meet Cresselia
If your talking about the lunar wing you must get the national dex. Then go to canalave city. go inside the house next to the sailor with the boat. the boy will be having nightmares. go talk to the sailor and he will take you to full moon island. cressilia will apear and run away. it will leave behind a lunar wing. to get cressilia you have to chase it around sinnoh. hope i helped.
first off get the lunar wing then return to the sailor and sail back to canalave. go into the house and give it to the boy. btw you do not get rewarded by anyone but you have a chance to catch the legendary cresselia.
To get Cresselia in Pokemon Platinum, you must beat the Elite Four, and then go to Canalave City. Below the library there will be a house, with a boy in the bed. When you talk to his parents, the Father will tell you that there is a Pokemon, capable of waking people up from nightmares. He will ask you to retrieve the Lunar wing, and will sail you to Full Moon island, when you talk to him. When you get there, speak to Cresselia, and it will disappear, leaving a lunar wing. After waking up the Captain's son, you will be able to find Cresselia roaming around like the three legendary beings.( Using the marking map app on the Poketch would be extremely helpful)
You don't get Ho-Oh's lunar wing, you get Cresselia's Lunar wing when you go to the island when the child of the sailor in canalave city falls sick...
After getting the Lunar Wing in Pokemon Diamond, Pearl or Platinum you can take it to Canalave City. In Canalave city is a NPC called Eldritch who is the same one who took you to Fullmoon Island. Giving Eldritch the Lunar Wing will wake his son from his nightmares.
in canalave city, go to sailor eldritch's house post hall of fame. his son is sick and he needs the lunar wing to make him better. he takes you there where you will meet cresselia. it runs off and leaves the lunar wing. sadly you have to relinquish the lunar wing now because the sailor needs it. now cresselia is flying around sinnoh like mesprit, awaiting capture.
you need to go to canalve city talk to the guy who takes you to iron island he will take you to a new island the follow the path to the Pokemon standing there he will fly away after you talk to it it will drop the lunar wing the go back you canalve and go to his house and use the lunar wing on his son in bed
go to canalave city talk to the sailor he'll take you to fullmoon island and there you'll meet cresselia and she'll flee and leave the lunar wing behind
Once you have the Lunar Wing in Pokémon White 2, you can go to the Marvelous Bridge and as you're crossing it, the wing will summon Cresselia so that you will have a chance to battle it and catch it.
It cant fly and it will die...
give it to the boy in bed by the deck
Get your National Pokedex and talk to the sailor in Canalave City. He'll ask you to sail to Fullmoon Island for a Lunar Wing to cure his son, put to sleep by Darkrai.
if you want to know how to make the kid better, the answer is this: you need the Lunar Wing. you get this magical item from cresselia on fullmoon island. then, with the lunar wing, talk to the boy in his bed. something will happen
It relates to the ghost in pokemon white 2/black 2 where she haunts the strange house and in it is the lunar wing and then cresselia will appear on Marvelous bridge if you have the lunar wing.
He does take it although you can't see it.