Reshiram is the Pokemon Black Version 2 specific legendary pokemon, originally the cover pokemon of the first Pokemon Black game.
To catch Reshiram in Pokemon Black Version 2, you first need to beat N’s level 70 Reshiram in N’s Castle near the end of the game to obtain the Dark Stone from him.
After you beat N, you’ll take the Dark Stone to Dragonspiral Tower. At the top of the tower, you’ll be able to use the stone to transform Reshiram from the Dragon Stone form back to its normal form so that you can battle and catch it.
