In its last few levels, 2001's Halo: Combat Evolved virtually turns into a survival horror game as you battle your way through hordes of twisted, undead enemies in dimly lit corridors, pressed to make every shot count and desperate to preserve your health as you press forward. Perhaps no level in Halo 's storied campaign presents such a sudden change of tone and intensity as "The Library," a long set of spacious corridors in which you first meet the sentient undead lifeform known as the Flood, and must face an almost inexhaustible supply of them while equipped with the game's most powerful weapons.