Reshiram's body was destroyed and changed into this stone. It is said to be waiting for the emergence of a hero.
This item did not appear in any previous games.
The Light Stone is a key item in Pokémon Black that contains Reshiram after it lost its physical form. In the final battle against N in his castle, the player will battle Reshiram and will get the option to put it directly in their party for the following battle against N. Its Pokémon White counterpart is the Dark Stone.