After you beat the Elite 4 you gain access to many new areas in Unova. Then you go to Lacunosa town if you talk to an old lady during the day she will tell you of a very scary monster that can generate extreme cold (it is Kyurem) she then tells you it lives in a chasm near the town. Once you make it there you go through the first cave and you will enter a forest area if you go the right way you will find a lake and once you go to it you hear a screech and every thing freezes then you go north and will find a cave and you go in and you find Kyurem. It is LV.75 so bring your best team and a lot of really good pokeballs.