Pokémon Diamond and Pearl/Fullmoon Island
Fullmoon Island is only accessible after getting the National Pokedex. Head to Canalave City and go to Sailor Eldrich's House next to his boat. You will find that his son is stuck in a nightmare, muttering about "...Dark...". This might mean Darkrai has done something to him. Talk to Sailor Elderich and he will take you to Fullmoon Island to help him retrieve the Lunar Wing to 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 which registers it in your Pokédex as seen. Afterwards Cresselia disappears, then roams Sinnoh like Mesprit. Cresselia will also drop the Lunar Wing where it was sitting so grab it and take it to Eldrich's son.
Note: The only reason to come back here is that one section of the Underground is inaccessible without it. You must not be in the glade where Cresslia is found, or else the Explorer Kit will not work.
Catching Cresselia[edit]
Grab a Pokémon that's between level 40 and 49 with Mean Look then head to the Route 205. Open the Marking Map Pokétch app then go on and off the route to the east(heading towards Valley Windworks) until Cresselia is on the same route as you. Once it's on the same route go into the grass put on Repel and wait to encounter Cresselia. Once you encounter it then immediately use Mean Look so it can't escape then use a sleep inducing move then switch to a Pokémon with False Swipe and get it down to 1. If it wakes up then put it back to sleep and repeat until captured. A good combo for this strategy is starting with Jynx by using mean look then using Lovely Kiss and switch to a Pokémon with False Swipe and switch back to Jynx to put it back to sleep when needed.
