So, it was 2001 and I was playing HALO for the first time on my new Xbox. My friend and I were going through co-op, and we did level 5 - ASSAULT ON THE CONTROL ROOM, which is one of the best single levels in gaming history. Then comes level 6, GUILTY SPARK. It starts in this rainy swamp, with this suddenly eerie music. It's nighttime, and right away there is a whole different feel to the level than in the previous ones. You push through the swamp, you enter the structure, and then one of the biggest and best creep out moments happens. The Flood.
I had no clue that game had horrible zombie monsters from another world.