The Hall of Serpents is located inside the Temple of Atal'Hakkar. It is full of bog beasts, oozes, maggots, worms, and a single Spawn of Hakkar. It is a circular hallway leading out to the six ledges containing serpentine statues that overlook the Pit of Refuse. It is accessed from the center-right and center-left passages upon entering through the instance portal, and by heading down the stairs rather than up.
Upon pressing the snake statues on those ledges in the proper order, the dire troll boss Atal'alarion spawns in the middle of the pit. The altar showing the proper order of the statues sits at the top of the staircase leading down onto the water-covered pit floor.