What 2007 Halo game first introduced the Flood pure forms added to Campaign Evolved? The answer is Halo 3. Released in 2007, Halo 3 expanded the Flood roster with new pure forms, specialized creatures that do not require human or Covenant hosts.
Earlier Halo games primarily showed the Flood as infection forms, combat forms, carrier forms, and other bodies created by infecting existing organisms. Halo 3 introduced pure forms as a further stage of Flood development. They could change between distinct shapes, including forms designed for combat, ranged attacks, and movement, creating more varied and visually unsettling encounters.
The pure forms were added to Campaign Evolved as part of its effort to incorporate later Halo mechanics and enemy varieties into a remake of Halo: Combat Evolved. Their inclusion acknowledges the broader series canon while expanding the original campaign's enemy encounters.
The date can cause confusion because Halo 2 also features the Flood and introduced major elements such as the Gravemind. However, Halo 3—not Halo 2, Halo: Reach, or Halo Infinite—was the first game to introduce the Flood's pure forms.