The Corrupted Sentinel is a named mob that has a chance to spawn after spending about 13 minutes in this instanced zone. If this named doesn't spawn, a PH "a stone sentinel" spawns instead.
It appears around /location -1300, -620 and roams the last hallway leading to the large open area above the pit overlooking the lower library.
It's snareable/rootable; single-target rampages; and flurries. It drops one stat item per kill, with a chance at dropping an armor piece as well.