

Interloper reintroduces docile Vortigaunts who won't attack you on sight, and also shows you how terribly they are treated by Nihilanth and its minions your first sighting of them on the level sees one being horrifically beaten by an Alien Grunt while weeping and they flee in terror once you introduce yourself.

Combined with a later radio broadcast revealing that an evacuation of New Mexico is now in effect, and it becomes clear you aren't just watching the defeat of the HECU - you're witnessing the beginning of the end for mankind. Even the intense final battle that occurs when Gordon reaches the surface is tinged with melancholy, as you can see a trio of Ospreys retreating alongside a HECU vehicle convoy, only for the latter to be vaporized by an alien aircraft. The distant tremors of air strikes can be felt throughout the level as HECU forces try to cover their retreat, and an air raid siren can be heard as you reach the surface.

Speaking of Surface Tension, there's a scene where you'll find a dying security guard begging for help until a scientist tries to save his life only to be shot by a HECU Sniper.(Guard screams, Alien Grunt savagely beats him)
