Nephalem Proving Ground 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 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.

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