Key mind

The Key Mind is an advanced form of Flood Gravemind.

Description
Key Minds are formed by the complete assimilation of planetary ecosystems by the Flood. They are larger and hold more intelligence than a regular Gravemind. They are capable of extraordinary strategic planning, such as controlling entire battle fleets, and are more than a match for metarch-class ancillas. The Key Mind basically a living planet composed of Flood biomass. Key Minds are distinguishable by their green and brown colors, as well as immense tentacles that flail around the creature. The atmosphere surrounding the Key Mind is composed almost entirely of Flood spores, and is extremely toxic.

Formation
When the Flood infects and assimilates every living thing on a planet, the individual forms merge together and form a sort of web over the entire surface of the planet. The Flood then proceeds to "digest" the planet, consuming the whole world and multiplying on a massive scale. This generates immense quantities of heat, causing the planet to literally "boil". The surface bubbles and rolls, while the atmosphere is superheated to temperatures high enough to melt any un-shielded vessel. The planet's local Gravemind then makes it's way to the core of the planet, where it assumes control of the whole mass. The newly formed Key Mind then sprouts immense tentacles all across it's surface. These tentacles are for absorbing solar radiation to provide energy to the Key Mind, as well as transmitting signals to other Flood minds. The tentacles can also serve as self defense.

Neurology
Key Minds by themselves have immense processing power, but when they begin to link up, they share their computational power, allowing the Flood to easily coordinate efforts across the Galaxy (or potentially many galaxies) in real-time. No species has ever been able to match the processing power of a Key Mind network.

Combat Strategy
The only known way to kill a fully grown Key Mind is to infuse the local star with iron, causing it to go supernova, vaporizing everything in the system. This was how Forerunners combated them.

List of appearances

 * Halo: Silentium