While playing Pokemon Black 2 and White 2, you can visit a town either called The Black City or The White Forest, depending which game your a playing. In this town, there is a special tower where you battle tough trainers.
Near the top of the tower (or tree hollow in White 2) there are multiple trainers with impossible to catch Pokemon. And each opener has certain Pokemon exclusive to the version you own.
For example, in White Two one trainer has a Thunderus in his party. Another woman has a Moltres and Raikou in her team. Then in White 2, there is also two bosses, Benga (the son of Alder I think) and Jariel. Benga's team in White 2 contains a Level 80 Latias, Dragonite, and Volcarona. Jariel has (get this) a Zapdos, Cresselia, AND a Latias ALL in his team.
Then in Black 2, one trainer has a Tornadus in her team. Another man has an Articuno and an Entei put in his team. Then the bosses are something. The bosses in Black 2 are Benga and Janna. Benga's team consists of Level 80 Latios, Garchomp, and, once again, Volcarona. Then Janna has a team of Suicune, Heatran, and Latios.
So, long story short, the Black Tower and the White Treehollow are some pretty tough challenges, and it'll make you flip tables trying to get through it. But let me tell you...it's all worth it.
Why?
Because after you defeat the entire tower, Benga will reward you with a Level 1 shiny Pokemon. Either Dratini or Gible (Dratini in White 2, and Gible in Black 2).
Personally, I think Gible is the better option because you can't get one anywhere else in the game, while Dratini you can catch outside the Celestial Tower.