Pokémon Diamond and Pearl/Fullmoon Island

Fullmoon Island is only accessible after getting the National Pokedex. You will find that Sailor Eldrich's son is stuck in a nightmare, muttering about "...Dark...". This means Darkrai has done something to him. Elderich will take you to Fullmoon Island, where a Pokemon, Cresselia, is said to reside. The Lunar Wing is the only thing that can heal his son. There is nothing on the island except a small glade where Cresselia is sitting in a pond. Talk to it to be shown a picture of it. Afterwards Cresselia disappears (like Mesprit).

The Lunar Wing is where it once was, so pick it up and heal Elderich's son. Note that the only reason to come back here is that one section of the Underground is inaccessible without it.