Halo Combat Evolved:
A standard FPS starring faceless Space Marine #103934802. Humanity losing a war against aliens, ancient superweapon appears, activate superweapon against aliens. Win.
Then you have the flood level. Cortana is left in the Control room, so for the first time you are alone in a freaky swamp. The standard rock soundtrack is replaced with ambient eerie music, and you find a downed abandonded human plane with a recording about how the crew were massacred. A little further in there's an alien dropship... also downed, something you've never seen before, and the scattered alien resistance is actually scared, you can hear them cowering and crying before you see them. Something is wrong, very very wrong.
Your motion tracker starts malfunctioning, showing allies where there are clearly none, you enter a mysterious structure in the swamp. the deeper you go inside it the more dead alien bodies you see. There's one human survivor, slumped againsta wall, driven mad he attacks you with a pistol babbling random nonsense. Most people probably left him alone, it would have been kinder to kill him.
You enter a narrow hallway covered from top to bottom in purple alien blood; no bodies or enemies, just gallons of blood. At the very bottom of the facility you come across a room, and then the cutscene plays:
And you suddenly realise you're all alone stuck at the bottom of the facility, standing in exactly the same room.
At this point if you pause the game the mission tracker in the pause screen stating your next objective just says one word: "ESCAPE" in all capitals. That was probably the bit that freaked me out the most, it was like they had invaded the very game itself, like they weren't supposed to be there.
