You can collect the Lunar Wing by interacting with Cresselia in the forest on Fullmoon Island. This will cause the legendary Pokémon to flee, dropping the Lunar Wing as it makes its escape.
Keep in mind that this can only be done after you’ve entered the Pokémon League Hall of Fame and received the National Pokédex upgrade.
Acquiring the Lunar Wing
You’ll need the National Pokédex upgrade before you can get the Lunar Wing, since you’ll be running into a legendary Pokémon that is absent from the Sinnoh Pokédex. Then you must visit the home of Sailor Eldritch in Canalave City.
Once your Dex is up to date, Fly to Canalave City and cross the bridge to the west of the Pokémon Center.
Pass the Canalave Gym and turn south.
Enter the southernmost house, which lies directly west of the docks (where Sailor Eldritch himself is standing).
Inside, you’ll find Sailor Eldritch’s son sleeping in his bed while his mother watches over him. Speak to her to find out what’s going on and trigger the next step.
Eldritch’s wife explains that their son refuses to wake up from a prolonged sleep.
Normally being a happy energetic child, Eldritch’s son is now bedridden and tormented by an endless nightmare.
She goes on to explain that this same nightmare has afflicted several Canalave citizens in the past, but that they were all able to wake up when exposed to a Lunar Wing from Fullmoon Island.
However, when her husband sailed out to the Island to search for one, he came back empty-handed.
At this point just leave the house and speak to Sailor Eldritch outside. When he asks if you’ll head to Fullmoon Island to look for a Lunar Wing, say Yes.
Sailor Eldritch will waste no time getting you to Fullmoon Island, which is a tiny landmass with a single linear path leading to a forest.
Landing on Fullmoon Island
When you arrive, head upstairs and circle around to the northeast.
Continue moving along the path until you reach the forest, then walk inside.
You’ll find yourself in a small clearing, where the Pokémon Cresselia will be resting Peacefully.
Walk up to Cresselia and interact with her. Your Pokédex will have just enough time to register Cresselia before the Pokémon flees the island (and begins roaming Sinnoh, ready for you to hunt and catch it).
As Cresselia leaves, she will drop a glittering item on the ground for you to pick up.
Interact with the item to add the Lunar Wing to your Bag.
Now you’re ready to head back to Sailor Eldritch to sail back to Canalave.
Lunar Wing Details and Uses
Once you’re back in Canalave City, you can take the Lunar Wing to Sailor Eldritch’s house and use it on his sleeping child.
Just walk up to the boy and press A, and the item will be used automatically.
The Lunar Wing will be consumed, and Sailor Eldritch’s son will snap out of his reverie.
Then Sailor Eldritch will enter his home, and the family will thank you profusely.
But there is another purpose to waking up Sailor Eldritch’s son aside from it being a moral imperative.
Once the boy’s curse has been lifted, you’ll be able to use the Member’s Card at the haunted Harbor Inn on the northeastern side of town. This will ultimately lead to a chance to catch Darkrai on Newmoon Island.