The Bound Keep

The Bound Keep is a secret area in Diablo III Adventure Mode, accessed only through a portal that can appear in any of the five Acts. If the portal appears, there will always be a bounty associated with it. There are four possible events to appear in this zone: Same set of events can also occur in Leoric's Gardens, Blighted Sewer, The Killing Grounds, The Archives or Nephalem Proving Ground.
 * A Plague of Burrowers
 * The Black King's Legacy
 * The Bound Shaman
 * The Cursed Shrines

Visually, it is a large combat arena in Keep Depths. Its layout suggests that originally it might have been a PvP battleground.