To get to the Pool jump down from the aqueduct that you are on when you enter the tombs and run to the back of the room and go up the ramp at ( -590, -4, 122 ) /waypoint -590, -4, 122. There is a climbable wall at top of the ramp to the left ( -619, 9, 110 ) /waypoint -619, 9, 110, climb it. Go into the water in the aqueduct turn right and jump off the edge. You are now in the Pool of Immortality. The pool can only be reached by jumping. In the pool are level 52 heroic Serpents of Immortality.