The Key

The Key is an Achievement for Halo 3 on the Xbox 360.

It is earned when one finishes the campaign mission Cortana on the Normal, Heroic, or Legendary difficulties, and gives 40 Gamerpoints to the player.

It is represented by a blue plate with a roman numeral "VIII" on it.

Note: You must finish the level completely through without quiting, or it won't count.