The final Diamond and Pearl Pokemon is Arceus, the actual god of all Pokemon, the creator of them all. While Arceus's Normal type may seem underwhelming at first, a quick look at its innate ability reveals the awesome power of this god among 'mon. By collecting special plates that litter the Sinnoh region and giving them to Arceus, you can change it to any type you want, allowing it to fill any number of possible roles in your team.
Arceus is an event-only Pokemon, so there’s no way to obtain one via normal gameplay.
Pokemon Diamond / Pearl / Platinum – Event only.
Pokemon Black / White – Event only. Most recently, an Arceus was given away in a Japanese-only Pokemon Global Link event shortly after B&W was released in Japan. Those Arceus's carry the individual players' OTs, so spotting hacks is tricky.
Trivia: Arceus has the highest base stat total of any Pokemon.
One of the three legendary birds from Generation I, Articuno is the Ice/Flying type third of the trio.
Pokemon HeartGold / SoulSilver – First, you'll need to obtain all 16 badges to make the legendary birds appear. Articuno can be found in the depths of the Seafoam Islands which you will need Surf to reach. Make sure you've got a Pokemon with the HM move Strength, as you'll need to move some boulders around to reach Articuno. He's roughly level 50, so make sure to bring an appropriate team.
Pokemon Platinum – Once you get the National Dex, the three Legendary Birds (Articuno, Moltres and Zapdos) will be roaming Sinnoh in the same way that Cresselia and Mesprit jumped around from place to place. Use your Marking Map Poketch app to see their current locations and track them down. Once the encounter begins, use a Pokemon with the Mean Look move to prevent each bird from fleeing (or better yet, use a Wobbuffet – its Shadow Tag ability will automatically prevent the bird from fleeing, even if you switch it out) and you should have all three in no time.
Pokemon FireRed / Leaf Green – You’ll find Articuno on Seafoam Islands. To get there, head south from Fuchsia City, and when you get to the ocean, use Surf to continue heading south until you reach Seafoam Islands. A level 50 Articuno is inside the cavern (make sure you also have a Pokemon who knows Strength in your party since you’ll need to move a few boulders to get to Articuno).
Pokemon XD – Articuno can be caught during the boss battle against Grand Master Greevil on Citadark Island. This is a pretty tough boss battle and it’s fairly difficult to catch the legendary birds this way. Since the bird trio is now available on Platinum and HG / SS there’s really no reason to go back to XD, unless, of course, you really want to.
Trivia: Articuno is the first legendary Pokemon in the National Dex.
Azelf is part of the Psychic legendary trio that debuted in Pokemon Diamond / Pearl. Each one guards one of Sinnoh’s three lakes.
Pokemon Diamond / Pearl / Platinum – Once you’ve beaten Team Galactic at Mt. Coronet, the lake trio of legendaries (Azelf, Mesprit and Uxie) will become available. You’ll find Azelf at Lake Valor.
Pokemon Black / White 2 – After completing the game, visit the Cave of Being and watch as the three Legendary fairy Pokemon fly to different locations around Unova. Azelf can be found in the Northern region of Route 23.
Trivia: Azelf (along with Mesprit and Uxie) has a cameo in Super Smash Bros Brawl in the Spear Pillar stage.
An adorable Psychic/Grass type Pokemon, Celebi is noted for being one of the most difficult Pokemon to obtain legitimately.
Celebi is an event-only Pokemon, so there’s no way to obtain one via normal gameplay.
Pokemon Diamond / Pearl / HeartGold / SoulSilver – Celebi was given away during the Pokemon Journey Across America Tour in 2006 with an OT of 10 ANIV, and again in 2011 during the B&W tour and at GameStops with the OT WIN2011. If you’re looking for a legit Celebi, your best bet is to seek out a reputable source for trading, like someone you know.
Trivia: Celebi is weak against a whopping seven different Pokemon Types (Bug, Dark, Fire, Flying, Ghost, Ice and Poison), making it the legendary with the most weaknesses.
Every generation of Pokemon games has a legendary trio. Black and White introduces a dual Fighting-type trio, dubbed the musketeer trio, and Cobalion is the first of the three you encounter in the game.
Pokemon Black / White – You can find Cobalion in Mistralton Cave after you receive HM03 Surf.
Pokemon Black / White 2 – Cobalion is found on Route 13, but after beating the Elite Four, it can be found here at a higher level.
Trivia: Not surprising given its legendary status, Cobalion's defense stat is the highest among all Fighting-type Pokemon.
Yet another Psychic-type legendary introduced in Diamond / Pearl, Lunar Pokemon Cresselia is the yin to Darkrai’s yang.
Pokemon Diamond / Pearl / Platinum – After beating the Elite 4 and obtaining the National Dex, head to Canalave City. On the west side of the city by the boat dock is a house with a sleeping boy in it. Talk to the boy, then take the boat across to Fullmoon. Follow the path (there’s only one way you can go) until you reach Cresselia, who will immediately flee when you approach. Make sure to grab the Lunar Wing she leaves behind, and bring it back to the sleeping boy to cure him. Now you’ll be able to find Cresselia roaming around Sinnoh (use the Marking Map Pokeketch app to view her current location).
Cresselia is probably the most difficult Pokemon to catch in Diamond / Pearl / Platinum, so you’ll have to employ some special techniques if you want to snag her without a major headache. You can take as much time as you’d like before you actually go after her (she’ll keep roaming around Sinnoh indefinitely until then), so take your time and make sure you’re well prepared.
-Using Fly to get closer to Cresselia usually doesn’t work. You’ll have to walk/bike there and hope he doesn’t move again while you’re en route.
-If that’s not working for you, find a boundary between two areas and move back and forth between them until Cresselia shows up in the area you’re in.
-If your lead Pokemon is lower than level 50, use a repel before entering the grass to ensure that you encounter Cresselia instead of a random wild Pokemon.
-Have a Pokemon with the Mean Look move at the head of your party (Crobat is a good candidate because of his high speed stat) and use it right away to prevent Cresselia from fleeing.
-Better yet, have a Wobbuffet at the head of your party. Its Shadow Tag ability will prevent Cresselia from fleeing even if Wobbuffet switches out.
-Cresselia’s HP doesn’t regenerate between encounters, so if you’ve gotten her health down and she flees, her health will still be down next time you encounter her. Be careful not to inflict poison because you don’t want to accidentally KO her.
If all else fails, Cresselia is an excellent candidate for using your Master Ball.
Pokemon Black / White 2 – Cresselia can be found on Marvelous Bridge after obtaining the Lunar Wing from the Stranger’s House.