How do you get the key for the iron chamber in Pokemon white 2?

In Pokémon White 2, you can obtain the key for the Iron Chamber by defeating the Team Plasma members in the giant chasm. After clearing them out, you'll receive the key from one of the Plasma members. Once you have the key, you can access the Iron Chamber, where you can find the legendary Pokémon, Reshiram. Be sure to prepare your team for the encounter!