This is a walkthrough for Fullmoon Island in Pokémon Brilliant Diamond and Shining Pearl (BDSP).
Fullmoon Island is a small island located north-west from the Sinnoh mainland. It’s only accessible during the postgame. Within, the Legendary Pokémon 
Cresselia can be found.
How to Reach Fullmoon Island¶
(1 of 2) Try to speak to the sailor’s son, who’s bedridden.
Try to speak to the sailor’s son, who’s bedridden. (left), The sailor will then request you to visit Fullmoon Island. (right)
After obtaining the National Pokédex, return to Canalave City and head over to the ferry at the west side of the city, near the south. Before speaking to the sailor, enter his house nearby and check out his sleeping son. Afterwards, when you speak to the sailor, he’ll beg you to visit Fullmoon Island and offers to take you there.
How to Get the Lunar Feather¶
(1 of 2) Fullmoon Island is small, with little of note.
Fullmoon Island is small, with little of note. (left), Besides this Legendary duck, of course. (right)
Upon arriving on the island, go around towards the east side, where there’s an entrance that leads further in. Here, you’ll encounter Cresselia. Interact with it and it’ll disappear, leaving behind a Lunar Feather. With the feather in hand, return to the arrival point and ask to travel back to Canalave City. Visit the sailor’s house once more and show the feather to the boy to wake him up.
How to Track Cresselia¶
After Cresselia leaves Fullmoon Island, it will begin roaming Sinnoh just like 
Mesprit. In fact, you can (and should) use the same method to search for and catch Cresselia. This includes grabbing the Marking Map app from the Pokétch Company building in Jubilife City.
Like Mesprit, Cresselia will start on a random route and will move to a different route when you change areas. Using HM Fly will reset Cresselia’s position, so don’t use it unless you don’t like its current position. It’s probably easiest if you and Cresselia are near a crossroad, like Jubilife City.
(1 of 2) We’re at Floaroma Town, while Cresselia’s at the Valley Windworks.
We’re at Floaroma Town, while Cresselia’s at the Valley Windworks. (left), Now we’re both on Route 205. (right)
Once Cresselia is in the same route as you, save your game! When you enter the tall grass or water, there’s a good chance you’ll run into the regular wild Pokémon. Should this keep happening, Cresselia may move to a different route, wasting your efforts. However, if you save beforehand, you can reload your save file and try again.
Alternatively, you can use Repels and lead with a Level 50 or lower Pokémon, but this may be inconvenient because…
Preparing for Cresselia¶
Similar to Mesprit, Cresselia will attempt to flee at the end of the first turn. If you don’t want to go on another wild goose chase, you’ll want some way of preventing Cresselia from legging it. The easiest way is to lead with a Pokémon that knows Mean Look or Block. We covered this in our guide for Mesprit, but we’ll go over it again just in case.
The most easily accessible Pokémon that fits the bill is 
Bronzong. We suggest catching a 
Bronzor in a Rocky Cave etc. and training it one Level, so it evolves into Bronzong and learns Block. If you have a spare Heart Scale, we recommend relearning Hypnosis. If you don’t have one, you could catch a lower Level Bronzor from elsewhere and manually train it up. Also, feel free to teach it attacking moves via TM.
(1 of 2) Here’s an Absol we caught from the Grand Underground with Mean Look.
Here’s an Absol we caught from the Grand Underground with Mean Look. (left), Just need to buy a Thunder Wave TM for Absol. (right)
If you’ve been grinding a lot (or don’t mind grinding), a more time-consuming but more efficient option is 
Absol. You can either breed it with an 
Umbreon or a 
Smeargle that knows/sketched Mean Look, or keep catching Absol in the Grand Underground until you’ve got one with Mean Look. From there, teach it False Swipe via TM. You can also optionally relearn Taunt, teach Thunder Wave via TM (gotten from the Small Sphere Traders or the Battle Park) and/or Swords Dance via TM (from Veilstone Department Store).
Legendary Pokémon¶
(1 of 2) Cresselia.
Cresselia. (left), Moonlight will be the biggest nuisance here. (right)
| Pokémon | Level | Type |
|---|---|---|
| Cresselia |
50 |  |
Cresselia is infamous for having absurd bulk, but abysmal combat abilities. Basically it’s one heck of a punching bag. It can use Slash and Psycho Cut for laughably low damage, Moonlight to heal itself (uh oh) and Safeguard to protect itself from status conditions. Like the Lake Trio, it has Levitate, which confers immunity to Ground attacks.
Immediately use Block or Mean Look to stop Cresselia from escaping. It doesn’t matter if your Pokémon is slower, since escaping has lower priority. That’s one problem solved. The other is weakening Cresselia and keeping its HP low. Which is fairly difficult considering its bulk and access to Moonlight.
That said, Moonlight can only be used 5 times, so you could simply live with it. Otherwise, if you brought along Absol, you can use Taunt to stop it using Moonlight and Safeguard. As an additional bonus, Absol is immune to Psycho Cut, although we can’t imagine Cresselia KOing anything.
(1 of 2) If you use Sword Dances 3 times, Absol will have max Attack.
If you use Sword Dances 3 times, Absol will have max Attack. (left), Afterwards, it’s False Swipe may reduce Cresselia’s HP by half. (right)
As mentioned above, Absol can also use False Swipe to safely lower Cresselia’s HP. But due to Cresselia’s high Defense, this might take a while. That’s why we suggested learning Swords Dance. Although doing so mean forgoing Taunt or Thunder Wave. In this scenario, we’d probably forget Taunt and rely on +6 False Swipe to slice away Cresselia’s restored HP.
If you don’t have False Swipe, you’ll have to make do with whatever moves your Blocker/Mean Looker has. The main concern is accidentally getting a critical hit, which is surprisingly easy if your Pokémon are friendly. Assuming you saved before encountering Cresselia, even if you KO it, you can reload and try again. Maybe even shuffle around your team, using the Box Link.
When the stars have all aligned–Cresselia has low HP and is paralyzed or asleep–there’s only one thing left to do. Yup, go ahead and throw your best/favorite Poké Balls at the lunar duck. You can’t go wrong with good old Ultra Balls, there’s Timer Balls when over 10 turns have passed and perhaps Dusk Balls if you encounter Cresselia at night.
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