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.
Underhell Chapter 1 + Underhell Prologue Remake
Do not install over the 2011 Prologue, first uninstall the old version, then install this clean.