This is a demo of Aboslute Zero mod. The mod is a recreation of early screenshots of Half-Life, the art-style and chapters featured off in Half-Life builds from early to mid 1998.

Playable Chapters that can be found in this demo:
Chapter 1: Anomalous Materials (a1a0)
Chapter 2: Unforeseen Consequence (a1a1)
Chapter 3: Office Warrens (a1a2)
Chapter 4: The Security Complex (a1a3)
Chapter 12: Communications Center (a3a1)

The authors are Cobalt-57

ModDB page