In Halo 3, the Flood intelligence that communicates with Master Chief is Gravemind.
Gravemind is the central intelligence of the Flood, a parasitic species that absorbs the knowledge and biomass of intelligent life. It speaks through a massive tentacled form and coordinates the Flood’s collective behavior across infected hosts.
The character first appears directly in Halo 2, where it captures Master Chief and the Arbiter. In Halo 3, Gravemind uses the Flood’s strategic power to oppose both the Covenant and humanity, while also manipulating temporary alliances when they serve its survival.
Gravemind is not an artificial intelligence. That distinction separates it from Cortana, Mendicant Bias, and 343 Guilty Spark. Its consciousness emerges from the Flood’s biological collective and is tied to the species’ highest stage of development.