The Proving Grounds is the final level of the Swampy Pit, an optional dungeon located in the Flayer Jungle. It is home to the boss Death Projector, which must be killed to get the Laser Focus Crystal charm.
To the south west of the room, there's red portal leading to Vizjun. To use it, the character must be at least level 125.
The Swampy Pit serves as testing grounds for Mephisto's latest siege weapon. The Death Projector slowly rotates, firing a devastating energy beam in a full circle, annihilating anything in its path.