Needler Sentinel
From Halopedia, the Halo wiki
The Needler Sentinel is a Forerunner Sentinel that can be found on the Halo 2 Campaign Level Quarantine Zone. It is a Sentinel Major that appears once at random places in the level Quarantine Zone. It shoots Needler rounds rather than the conventional Sentinel Beam and when destroyed, drops a Needler.
The Needler Sentinel can be found on the leftmost side of the first glowing orange indoor area within the ruins of the Sentinel Constructor Factory.