Nephalem Proving Ground is a special area in Diablo III Adventure Mode, accessible only when there is a bounty associated with it, which can occur in any Act. It is reached through a portal that appears somewhere in that act. There are four possible events to appear in this zone:
The same set of events can also occur in Leoric's Gardens, Blighted Sewer, The Killing Grounds, The Archives or The Bound Keep.
Visually, it is a large combat arena in Nephalem ruins. Its layout suggests that originally it might have been a PvP battleground.
Entrance portal
Central part
