Stalkers (Half-Life)

Stalkers are minor antagonists in the Half-Life series. They are humans who have been physically and mentally twisted by the Combine to serve as mindless slaves within the Combine Citadel.

Half-Life 2
Combine Stalkers are first seen when Gordon Freeman is venturing into the Citadel, where they are manufacturing Combine weaponry and are screaming repeatedly as they are moved on a skylift.

Half-Life 2: Episode One
They are later seen and must be avoided (otherwise they could attack) in the Citadel, still mindlessly operating machinery, prior to its destruction.