The video I'm showing today is another oldish one I made when testing the code/map components for porting in monsters through The Gate.
It's one of the key gameplay elements of the mod. As the mod takes place within one very large map, most of the enemies the play will face are not present as you start the game. They will arrive as the story progresses, triggered by events, previous monsters being killed etc. It will be similar to Horde in Gears of War 2 and Firefight in Halo, but with more of a narrative flow and context.
In the video above I just set up a simple trigger in the map to cue the monsters's arrival; in the game it will be triggered in .qc code. In the next update I'll share some of the code behind this, how the timings, monsters are configurable etc., depending on what stage of the mod the player is at.