i am trying to make a thing, you might say, were your walking down a hall and you see a turnoff, and you can't go there, and there is combine soldiers running by in the part you can't get to. like in the a level of nova prospekt except they do it with antlioons (p.s. when they get out of sight i need them to disapear)

is this done with scripted sequences or what?
can someone please tell me how to do this. please.