As soon as you descend into the ruins of N's Castle, you'll run into none other than N himself! He'll say a bunch of stuff and then will ask if he can follow you around, which you should say yes to.
There aren't any wild Pokemon in here, unfortunately, but you can enter the room just to the north of the entrance to find N's old room, which he will talk about if you bring him inside. There's a Luxury Ball over by the train set, but that's about it.
Out in the main hall, you won't be able to head straight east, so climb down into where the water once flowed, picking up the Protein over to the west before heading east.
In the eastern-most part of the room, down one of those little ledge things, you can find TM50 (Overheat), a powerful Special-base Fire-type attack that lowers the user's Special Attack by two stages afterwards.
Climb up the stairs and follow the path forward until N stops you, saying you should go forward alone. Keep going and you'll enter the room where the you fought Reshiram/Zekrom, N, and Ghetsis in the original Black and White, except it's in shambles.
Head forward and N will follow after you. After some talking, he'll release his Zekrom (Black 2) or Reshiram (White 2) and will then ask if you want to battle. If you're ready to take him on, say Yes, otherwise say No and you'll be given a chance to heal and save.
N uses Zekrom if you're playing Pokemon Black 2 or Reshiram if you're playing Pokemon White 2. He does NOT use both on his team.
Even though he uses only one Pokemon, it's an uber legendary Pokemon and is very strong, meaning you will be in for one heck of a fight, especially if you haven't done much training after beating the game.
Catching a Pokemon like Excadrill can help a lot against Zekrom, because it is immune to its powerful Fusion Bolt attack while resists its other attacks, Zen Headbutt and Dragon Claw. You may have to train it up a bit, though.
Regardless of which one you have to fight, getting TM26 (Earthquake) on Route 15 is very helpful, as is any Dragon-type attacks you may have. Be careful about using your own Dragon-type Pokemon, because his are both fairly quick and their high level doesn't make it a walk in the park.
After beating N in his castle, you'll be able to run into him in the Ferris Wheel on Fridays in Nimbasa City. Once you've ridden with him on the Ferris Wheel, he'll return to his castle, where you'll be able to battle him.
He uses a different team depending on which season it is, with levels ranging from level 75 to 77, making him very tough to take down. You can only fight him once per season.
After beating him, he'll talk for awhile and eventually return the legendary Pokemon to its stone. This means he'll return Zekrom to the Dark Stone in Black 2 and Reshiram to the Light Stone in White 2. He'll give you that stone afterwards, which you then need to bring back to the Dragonspiral Tower north of Icirrus City.
While you're visiting Opelucid City, you can return to Drayden's house in the middle of town with just about any Dragon-type Pokemon to have him teach it the powerful Dragon-type Draco Meteor attack. He will teach it for free and as many times as you like, so feel free to reward your dragons with a powerful attack! Just keep in mind that it lowers the user's Special Attack by two stages afterwards, but it can be a nice attack on Dragon-type Pokemon with high Attack so they can keep attacking afterwards.
Return to the Pokemon League and then head south onto Victory Road. Enter the nearby cave and keep following the path until you find the two Veterans. South of them is a small staircase leading down. Take it until you see the Zoroark in the bottom-right corner of the screen, which is where you should head on over to.
Follow the path through the cave until you wind up on the other end, where you'll run into the Zoroark again. If you came this way before beating the game, that Zoroark stood in the way of the cave there, but now that you've beaten it, it'll lead you inside.
Inside, it will lead you over to a staircase which takes you down to N's Castle, in ruins and everything.
Ah, good old Tubeline Bridge. You can finally access it now and head westbound (although it will look like you're traveling south due to the camera angle).
The Battle Girl walking in circles a little south from the entrance will give you TM43 (Flame Charge) after talking to her. Flame Charge is a weaker Physical-based Fire-type attack that raises the user's Speed by one stage afterwards. It only has 50 power, though, so it won't do a lot of damage, but it can be helpful for some Pokemon.
In the gate on the other side of the bridge, talk to the Scientist and he'll give you an EnergyPowder. Uh, ok. That's not going to do you all that much good, though, but whatever.
As soon as you enter the route, talk to the Parasol Lady and she'll give you an item depending on what time of day you talk to her. She'll give you the Damp Rock if you talk to her in the early morning, the Heat Rock if you talk to her in the daytime, the Smooth Rock if you talk to her in the afternoon and early evening, and the Icy Rock if you talk to her at night. She'll give you only one rock per day, though, and only one of each rock. These extend the duration of the respective weather condition from 5 turns to 8 turns instead.
There's a Big Pearl that requires Surf in order to reach by the Fisherman.
Heading to the west of the Fisherman will lead you along a wet path. In the northwest corner, you'll find a Youngster using a Palpitoad L60, a Croagunk L60, and a Vaporeon L60. To the north of him is TM36 (Sludge Bomb), a decent Special-based Poison-type attack with 90 power and a chance of inflicting poison. Over to the east of that is a Nest Ball.
Directly south of the Youngster, across a puddle of water and in the middle of some flowers, is a hidden Rare Candy, which you should definitely pick up. Keep going east and you'll find the entrance to Icirrus City!
Right near the entrance to the Moor of Icirrus, over to the left, is a hidden TinyMushroom. There's also an old guy to the north that will tell you a bit about the history of Keldeo, but that's it. If you have Keldeo in your party, he'll tell you a little more about the story.
Push a nearby boulder with Strength and you'll find a little lake area. Fight the PKMN Ranger nearby, who uses an Accelgor L62, a Kecleon L62, and a Swalot L62. She'll give you a Lum Berry after beating her. Head south of her (if you have Surf, you can just cross the water; otherwise, you'll have to take the long way around it) and you'll find TM84 (Poison Jab) sitting there for you to take, which is a fairly strong Physical-based Poison-type attack.
Return back to the old guy, except this time head north and then a little west. Up there, you'll see a Fisherman, but first walk over to the west and pick up a hidden Big Mushroom nestled into the trees. The Fisherman has a Poliwhirl L60, a Politoed L60, and a Whiscash L60. Over to the east of him is a Green Shard.
Head back towards the old guy, except go east instead of west where I told you to earlier (so northeast of the old guy) and keep going northeast until you find a Heart Scale. Pick that up, then head back and east instead of northeast, jumping down a ledge. Down there is another Fisherman, this one using a Corphish L60, a Poliwag L60, and a Stunfisk L60. There's a little patch of flowers southwest of the Fisherman which have a Dawn Stone hiding in them, which can be used to evolve certain Pokemon such as male Kirlia or female Snorunt. There's also a Rare Candy in the trees in the middle of that little island.
South of that island is a PKMN Ranger using an Escavalier L62, a Skuntank L62, and a Carnivine L62. You'll get a Lum Berry from him after winning. Grab the Full Heal near him, then head west to reach the entrance. That's all to see here, so go on and return to Route 8 to make your way to Icirrus City!
You'll first arrive on the eastern side of Icirrus City, over by a windmill. Head south and you'll find an inaccessible house, blocked due to a ledge.
Ramping up for Winter
The house in the southeastern part of Icirrus City is only accessible during the winter months.
Up the stairs and then across the bridge to the east, you'll find a lone item sitting there for the taking. However, when you approach it, the Shadow Triad will suddenly appear and ask if you want to battle! Yikes! You'll have the option to say No just in case you aren't fully healed, and since you'll have to fight all three in quick succession, you should make sure you're healed before taking them on. You can fight them once per season.
You'll fight the first one in a simple Single Battle and he shouldn't be too challenging to take down, as his Pokemon's levels aren't too much higher than some of the Trainers you've been fighting.
Afterwards, you'll have to fight the second member of the Shadow Triad, except this time in a Triple Battle. You'll be given the choice to battle them or not, though. If you say No, they'll both disappear and leave you free to pick up the item without fighting them for the rest of the season.
Being a Triple Battle, you'll want to make sure you have a sturdy Pokemon in the middle. A Fire-type is your best bet, but a solid Fighting- or Ground-type Pokemon should sit there comfortably.
After beating the second one, you'll be left only to fight the third and final member of the Shadow Triad, who will also ask you if you're up to battling. Again, if you say No, he'll just leave until the next season, where you'll fight all three again.
The third of the trinity will fight you in a Rotation Battle, meaning you may run into trouble if you're looking to use Fighting-type attacks until you get rid of that pesky Banette, who is completely immune to them. Once it's out of the way, though, you can use Fighting-type attacks to easily defeat those Bisharp. Otherwise, Fire- or Ground-type attacks are safe bets against all three.
After they're all out of the way, you'll be able to reach the PP Up, which hardly feels like a fitting reward for taking out Ghetsis' minions, but ah well, it never hurts to have a few of those. You're able to fight them again each season by visiting the same spot.
Far to the west of that, across a bridge or two, you'll find the Pokemon Center. Inside, talk to the Battle Girl with the Cubchoo and she'll give you a Black Belt, which pumps up the power of your Pokemon's Fighting-type attacks. The Gentleman by the counter will buy any of your minerals, gemstones, and other stuff like that for far more than you'll get at the Poke Mart. You can sell your Nuggets for $10,000 apiece through him, and your Big Nuggets for a whopping $30,000! He'll also buy things like the Gems, any evolutionary stone, Star Pieces and Stardust, and so on.
The first house to the north of the Pokemon Center has nothing of interest, but the one just past it has three people in there that will rope you into answering a question for a quiz. You can do this each day, but the prizes aren't that great (Antidote if you win, Parlyz Heal if you lose — you're arguably better off getting it wrong).
The Icirrus Gym is still around, although it's entirely empty, except for the ex-Gym Leader at the end. Brycen can be found all the way in the back just like in the old games, except he won't fight you, but will instead talk about Alder and some stuff before leaving. Oh well!
Memory Link: Brycen's Mentor
If you've performed a Memory Link with your original Black or White version that's already beaten the game, you'll unlock a Memory Link scene showing Brycen, Alder, and Marshal all talking about Alder retiring from being the champion.
The house south of the Gym is a Pokemon Fan Club, and you can show the girl inside one of your Pokemon and she'll tell you how high its happiness is towards you, while you can show the chairman inside one of your Pokemon and he'll give you an item based on how many levels it has gained since you first acquired it.
If it's gained 25 to 49 levels from when you first caught it, he'll give you an Exp. Share.
If it's gained 50 to 98 levels from when you first caught it, he'll give you a Cleanse Tag.
If it's gained 99 levels from when you first caught it (usually requiring training a Pokemon hatched from an Egg to level 100), he'll give you a King's Rock.
That's all there is to do in Icirrus City. If you didn't visit in the winter time, you'll definitely have to come back when it's winter, as there's a few things you can only get during the wintertime.
There's an entrance to Twist Mountain in the southwest part of town, although the place you'll likely want to check out the most is Dragonspiral Tower to the north, which should be your primary destination so you can catch yourself either a Zekrom (Black 2) or Reshiram (White 2). (Besides, there's more you'll be able to do in Twist Mountain after exploring Clay Tunnel, so that's where I'll cover it.)
Oddly enough, there's a little walkway just outside of... the outside of Dragonspiral Tower. Yeah, it's weird, but it does have slightly different encounters, plus there's a ramp made out of snow that appears only during the winter which leads you to a NeverMeltIce (raises the power of Ice-type attacks when held).
As you can see, there are quite a few Pokemon to find here! If it's winter, you'll run into a few different ones. Anyway, when you arrive at the foot of Dragonspiral Tower, you'll find Cedric Juniper there and he'll talk about the events two years ago. He'll then notice your Dark Stone or Light Stone, say some more stuff about it, and then give you some Old Gateau before leaving. Yum.
You can waltz right on into the tower if you want, but if you have Surf handy, you can swim on over across the pond to pick up a Carbos, as well as the tall and thick grass.
The first floor is pretty straightforward. Extremely straightforward. Just follow the path and you'll find some stairs. Cool!
Dragonspiral Tower 2F
Up here, you can save some time if you have Strength, but otherwise you'll have to take the long route. Either way, you'll have to head north. Pushing the boulder out of the way leads you to the stairs, but if you don't have Strength handy, you can still get there by continuing north, picking up the hidden Red Shard in the rubble as well as the Elixir underneath one of the fallen columns.
Keep going until you find a broken pillar you can climb on the far right side of the room, then walk across it until you reach the stairs where you would have arrived had you pushed the boulder. Before continuing, pick up the Ultra Ball sitting on a broken column.
Dragonspiral Tower 3F
In the next room, you'll climb up some stairs before finding a series of one-way ledges that you can jump across. Jump down the one leading south and you'll be presented with three ledges you can hop across.
Ignore the top ledge. The bottom one will take you to where you want to go, but taking the middle one leads you over to a Shiny Stone, which is always a nice thing to pick up so you can evolve some of your Pokemon. Keep jumping along to the north and then east until you find yourself on the same platform as a Protein, which is, once more, a decent item. Hopping south will lead you back to the beginning.
Back at the beginning, take the bottom ledge and follow it until you reach the platform with the stairs leading up. Assuming you picked up the items listed above, there's nothing more for you to worry about here, so just head on up to the next floor.
Dragonspiral Tower 4F
This floor is fairly straightforward. Just follow the path up all of the stairs until you find a fork in the road, with stairs leading up and east. Swing by to the east to pick up the trio of items for your Sinnoh Legendary Pokemon: the Adamant Orb, the Lustrous Orb, and the Griseous Orb. Keep in mind that you can't actually catch Dialga, Palkia, or Giratina in Black 2 or White 2, so you'll need to transfer them from a previous version (or obtain them from the Pokemon Dream Radar app for the 3DS while you have Diamond, Pearl, or Platinum inserted into your 3DS).
Now head up the stairs to the next floor.
Dragonspiral Tower 5F
Ah, the cylinder room. This one is interesting because the whole room will move along with you (well, at least the camera will, making it seem so).
Start by heading east and then across the bridge to the third ring from the center. Head west and across the bridge to the second ring to pick up the mighty TM02 (Dragon Claw), which is a reliable 80 power Physical-based Dragon-type attack that a variety of clawed Pokemon can learn.
Hop across the ledge and head east until you run into the bridge leading into the center ring. Pass by it for now and walk a few more steps to the east until you see the two bridges right next to each other. Walk across both of them and then west to pick up the PP Max, which is a rare item, indeed, that's the equivalent of 3 PP Ups.
Alright, now return to the bridge leading to the center ring and cross it. From there, take a right and jump across the ledge, then cross the bridge to reach the second ring from the center. Follow the path until you're at the third ring from the center, by the bridge leading to the outermost ring. Keep going east to pick up a Max Revive (them dragons keep a lot of good items here!), then go back and cross the bridge, heading west until you reach the stairs leading you up to the next floor.
Dragonspiral Tower 6F
Again, this is a rather straightforward floor. Just follow the path north and pick up the valuable Comet Shard, which can be sold to the Ore guy in the Icirrus City Pokemon Center for a whopping $60,000! Very nice, indeed. Nothing else though to worry about on the floor, so keep going until you reach the stairs up to the top floor.
Dragonspiral Tower 7F
Ok, this is it! Walk across the narrow pathway and over to the middle of the room. The whole place will shake and you'll be presented with a question asking whether or not to use the Dark or Light Stone (depending on which version you're playing). If you answer Yes, you'll summon the legendary Pokemon that N used — either Zekrom (Black 2) or Reshiram (White 2) — although you won't have to fight it right away. You'll be presented with a cool cutscene, which is mostly the same one from the original Black and White, although with the background set in Dragonspiral Tower instead of N's Castle.
Why isn't Reshiram/Zekrom appearing?!
If you make it all the way to the top of the Dragonspiral Tower and you find that nothing's happening up there, that means you haven't gotten the Dark Stone or Light Stone from N in N's Castle. You'll want to return to Victory Road, where the Zoroark is (it's down the eastern path after heading south from the Pokemon League entrance), follow it to N's Castle, and then fight N there. Only after defeating him will you have the Dark Stone or Light Stone necessary to summon Zekrom or Reshiram respectively.
Before fighting the legendary Pokemon, you're going to want to make sure you save your game! Otherwise, you'll have to defeat the Elite Four and Champion again in order for it to reappear, and they use much, much stronger Pokemon after beating the game the first time. Additionally, you might as well sneak around the legendary Pokemon and pick up the hidden Comet Shard in the northeast corner of the arena. I told you them dragons know how to live!
While Reshiram and Zekrom may be at a high level, they thankfully have a very manageable catch rate, making them easy to catch. Opening the fight with a Quick Ball may fetch immediate results, but otherwise strike it with Ice-type attacks for Zekrom, Rock-type attacks for Reshiram, or Ground- or Dragon-type attacks for either of them. They should be sturdy enough to survive at least one hit from those attacks, although you may have to use a weaker attack afterwards to lower their HP.
Anything less than around 40% HP should be fine enough to catch it, even with an Ultra Ball, although if you can wait ten turns out, you'll find that Timer Balls are your best bet. Keep in mind that both have Imprison, which will prevent your Pokemon from using any of their attacks — that means no Dragon Claw while fighting Zekrom and no Dragon Pulse while fighting Reshiram, assuming it uses Imprision on you. It's not a huge deal, but it may cause you trouble if that's what you were planning on doing.
As before, Excadrill is a decent Pokemon to use against Zekrom, as none of its attacks hit that well. Tyranitar (which you can catch on Route 15, preferably as a high-level Pupitar and then level it up one level) also works against Reshiram, but you have to be careful due to Sand Stream whittling away its HP each turn.
After catching Zekrom or Reshiram (or knocking it out), N will appear and will tell you to return to the Giant Chasm to find another of his 'friends.' He'll then head off, back to his castle, where you can fight him once per season after you've run into him on the Nimbasa City Ferris Wheel (although he is very strong, so you aren't likely to be able to win against him just yet!). He uses a different team depending on the season, too.
Well, now that you've got your legendary Pokemon, you can use Fly on the top floor to escape. You should return to Lacunosa Town, so you can make your way back to the Giant Chasm. (You did activate the shortcut to the Giant Chasm with Strength, right? Otherwise, get there from Route 22.)
Return to the foresty area of the Giant Chasm to see that it has mostly recovered, except for the sandy "crater" in the middle. Head to the eastern side of the forest, then north to reach Kyurem's cavern.
All the way back in the room where you fought against Black Kyurem or White Kyurem and Ghetsis, you'll find Kyurem, just sitting there, waiting for you. Make sure you save your game before fighting it, otherwise you'll have to wait until after you've beaten the Elite Four and Champion again for it to reappear, and they're pretty hard!
Unlike Zekrom and Reshiram, Kyurem has the standard catch rate of a legendary Pokemon, meaning it's likely to be a pain to catch. Dusk Balls are most effective against it for the first several turns, thanks to being in a cave, while Timer Balls are more effective after ten turns have elapsed.
You'll need to weaken it as much as possible in order to have a fair chance of catching it, and past that, it all boils down to luck, so you may catch it in your first Dusk Ball or it may take thirty (or more!). I recommend bringing about 10 Dusk Balls and 30 Timer Balls just in case.
Any Steel-type Pokemon (that isn't part Ground-type) should have no trouble walling Kyurem's attacks. It also has only two attacks that actually do damage, so you're at less risk than you were fighting Zekrom or Reshiram. Speaking of which, Reshiram is a particularly good Pokemon to use against Kyurem, particularly if you lead with Imprison to prevent Kyurem from using Dragon Pulse. Just be careful, as Reshiram's Dragon Pulse can possibly do enough to knock it out (although it will usually come close, which is good).
After catching it (or knocking it out), Cheren and Bianca will appear, say some stuff, and then leave. Make sure that you pick up the DNA Splicers that appeared where Kyurem once stood, which is the item you use to fuse Kyurem with either Zekrom or Reshiram!
In order to get yourself either Black Kyurem or White Kyurem, you're going to need to have both Kyurem and either Zekrom for Black Kyurem or Reshiram for White Kyurem, as well as the DNA Splicers item found after catching/defeating Kyurem.
You need to have both Kyurem and Zekrom/Reshiram in your party in order to fuse them. Use the DNA Splicers from your items (it's a Key Item) and you'll first need to select Kyurem, then you will need to select Zekrom/Reshiram for the second Pokemon. It will then fuse them together to create either Black Kyurem or White Kyurem, removing the other two Pokemon from your party (so you'll have an empty spot).
The fused form of Kyurem takes on the level, nature, and other details (like Effort Values and Individual Values) of Kyurem. Its moves may vary depending on which TM moves you've given it in either its Normal Kyurem form or Black/White Kyurem form, so keep that in mind when you fuse it or separate it.
You cannot use the DNA Splicers to fuse Kyurem if you already have another Kyurem form present, meaning you can't have both Black Kyurem and White Kyurem together. You also can't trade White Kyurem or Black Kyurem and need to defuse it first.
Additionally, after you've caught Kyurem, the next time you use Amanita's PC, you'll receive some special Kyurem wallpapers. Nifty!