Forerunner automated turret
From Halopedia, the Halo wiki
The automated turret is a form of equipment so far only encountered on the Halo 3 level The Covenant (level). It resembles a stripped-down sentinel, and fires a thin, blue, sentinel-beam-like beam at enemies. However, due to programming issues, it has major problems picking out targets and more often follows allies with its targeting system, and does not fire.
