Just a little hint I guess,
After you beat the game, the rival's sister tells you she heard about a Zoroark on Victory road. If you go there, Zoroark will retreat into the cave upon seeing you. Follow it, and it will lead you to N. N will join you and lead you to the ruins of his castle where the dragons battled it out 2 years ago. He will then challenge you to a battle with his dragon (Zekrom in Black 2, Reshiram in White 2). Beat him, and he'll give you his dragon stone. Take it to Dragon Spiral Tower. It will awaken and you get to battle it (it's level 70). Catch it, and N will appear and take note of your battle, and will disagree about you keeping it in the ball.
After you beat the game, the rival's sister tells you she heard about a Zoroark on Victory road. If you go there, Zoroark will retreat into the cave upon seeing you. Follow it, and it will lead you to N. N will join you and lead you to the ruins of his castle where the dragons battled it out 2 years ago. He will then challenge you to a battle with his dragon (Zekrom in Black 2, Reshiram in White 2). Beat him, and he'll give you his dragon stone. Take it to Dragon Spiral Tower. It will awaken and you get to battle it (it's level 70). Catch it, and N will appear and take note of your battle, and will disagree about you keeping it in the ball.