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.

AshHousewares says

10/10 - Agree (3) Disagree

I've never seen a mod quite like this before. From the start, I was immediately immersed into the world of Underhell. It feels so refreshing to see a mod like this come about. I can't wait to see what happens in the next chapters. What a fantastic mod. Shouldn't be missed to whoever has HL2 installed :D.