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.

Gordon101Gordon says

10/10 - Agree (2) Disagree

Simply put, THE BEST HALF LIFE 2 MOD I PLAYED.
It has a good story, good mapping, gameplay, etc that anyone would like. It also has the really good puzzles at The House.