Dark Engine

Information from wikipedia:

The Dark Engine is a 3D game engine developed and used by Looking Glass Studios for Thief: The Dark Project (1998), Thief II: The Metal Age (2000), and the Irrational Games joint projects System Shock 2 (1999) and Deep Cover (the project was cancelled).

Source code is based on Thief 2 leak. Don't use it for commercial purposes.