In the back of the Cave floats a "mysterious ring". The player may interact with this ring to battle ReshiramOR/ZekromAS. If defeated or run from, it respawns after entering the Hall of Fame.
The music that plays when battling Reshiram or Zekrom is the theme that plays when battling Zekrom in Pokémon White and Pokémon Black 2, despite Reshiram having its own version of the theme in said games.