Black Kyurem is one of 2 alternate versions of the Legendary Pokemon, Kyurem (alongside White Kyurem). This alternate form debuted in Pokemon Black Version 2. After catching both Zekrom and Kyurem, this form can be accessed by using the DNA Splicers to fuse the 2 Legendaries together, granting a signature move that is exclusive to this form. This process can also be reversed whenever you want to turn them back to normal. However, in the anime, Kyurem is able to transform into either of his forms at will, without the need for Zekrom, Reshiram or the DNA Splicers. The lore states that the Legendary Dragon Trio were once one single all powerful Legendary Pokemon, who allied with 2 twin heroes, who sought a world full of truths and ideals respectively. But they couldn't settle on one or the other. So, the Legendary Pokemon somehow split into two dragon type Legendary Pokemon called Zekrom and Reshiram, each one siding with one of the heroes. Also, legends say that a third dragon type Legendary appeared as well. One who controls ice, and produces a harsh freezing wind from it's frozen body. A pokemon that is practically the leftover shell of its former self, waiting for someone to fill the empty parts of its heart, mind, body and soul with either truth or ideals. That pokemon is known as... Kyurem! Also, it is unknown what the three of them looked like before they split apart.