Gravemind (Earth-2042)

"I am a monument to all your sins."

- The Gravemind

The Gravemind, infrequently referred to as the Ancient One, and formally known as a compound intelligence, is the shared consciousness of the entire Flood species which first emerges when the Flood reaches the Coordinated Stage. While this compound mind inhabits all Flood forms, it is distinctly embodied in processing nodes of agglomerated Flood biomatter known as key minds. The Gravemind is the highest stage of the Flood's development, though its complexity and capabilities depend on the size and number of the key minds which can, at the height of a Flood outbreak, engulf entire worlds.

Source

 * https://www.halopedia.org/Gravemind