The Black King's Legacy is a special event in Adventure Mode of Diablo III. It can only appear if nominated a bounty, in any zone of any Act. If it appears, one can go through a red portal and access any of the secret zones: Leoric's Gardens, Blighted Sewer, The Killing Grounds, The Archives, The Bound Keep, or Nephalem Proving Ground.
The event itself requires cleansing a Cursed Chest. Once it is touched, a timer will start, and 3 Champion Packs will appear one by one. All three must be killed before time is up to receive a bonus Radiant Chest.