Underhell is a Source Engine game developed by the Underhell Team in collaboration with We Create Stuff. Underhell Chapter 1 requires ONLY Source SDK Base 2007 installed. No Source game is required.

gdp002 says

10/10 - Agree (2) Disagree

This mod is bloody awesome, I advise you not to start with the prologue chapter itself, and instead, try and solve the mysteries of the house.
The atmosphere adds a bone chilling effect to the plot, intensifying the player's actions while it progresses.