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The Killing Grounds are 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 events to appear in this zone:

Same set of events can also occur in Blighted Sewer, Leoric's Gardens, The Archives, The Bound Keep or Nephalem Proving Ground.

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