Halo. 343 Guilty Spark level. It's the one where you first encounter the Flood. After going through half a game fighting aliens, I'm suddenly dropped in the middle of a swamp. I get aliens that used to have no problem attacking me, running and not giving me a second look. A crashed dropship, dead marines that weren't killed by the aliens, evidence of firefights that covered--and destroyed--entire rooms, no live Elites, a crazy marine who's mumbling to himself, and finally that video clip you find. And to top it all off, after trying desperately to kill the little spores, to turn around and suddenly find a full Flood charging me...yeah. The first time I played that level, I was a little unnerved. Loved it though.