After defeating the Champions League and entering the Champion's Room you will find N and discover that he has just defeated the Champion himself. He will then command his castle to rise from the ground and suddenly N's gigantic castle will appear and attach itself to the Pokémon League building, completely engulfing you. N will disappear to the top of the castle and challenge you to reach him and see if you can beat him and his new weapon.

As you enter the castle the 6 Sages will attempt to block you off, only for the Gym Leaders of Unova to turn up and protect you. While they take care of the Sages, you can explore the castle. Watch out for a room on the 2F which contains a lady with pink hair. If you talk to her she will heal your Pokémon for you! And trust me, you'll want that.

Eventually the Dark Trinity will reappear and point you in the direction of N's room, where he grew up as a child. It's a rather nice room actually, and you can find yourself a valuable Rare Candy inside on the half-pipe. And I never thought I'd ever type that.

When you reach the 5F Ghetsis will appear and stop your progress, before ultimately stepping aside. Enter the room and N will talk to you before ultimately summoning either Zekrom (Black Version) or Reshiram (White Version)! All may look lost but your Light Stone or Dark Stone will begin to glow and will summon your very own legendary Pokémon - Reshiram in Pokémon Black or Zekrom in Pokémon White! Make sure you use this opportunity to save your game before you get the chance to capture the wild legend!