The Director

The artificial intelligence of Left 4 Dead features a dynamic system for game dramatics, pacing, and difficulty called the Director. Instead of set spawn points for enemies, the Director places enemies in varying positions and numbers based upon each player's current situation, status, skill and location, creating a new experience each playthrough. The Director also creates mood and tension with emotional cues such as visual effects, dynamic music and character communication.

The Director can adapt to any existing map, including those initially developed without Left 4 Dead's event markers, as seen in this video of the Counter-Strike: Source map de_dust running in the demo: http://www.youtube.com/watch?v=_SlXeiRH8VI