They are transsentient beings that manifest in our reality when "corpses" are gathered together to form a compound mind.

What is transsentient? I haven't been able to find a definition for the word.

It means they are beyond sentient.

Interesting. I know the Gravemind is essentially omniscient, but I've never heard the word transsentient used to describe it or anything else. I honestly don't even know what 'beyond sentient' means since I don't know how anyone or anything can be beyond a state of sentience , unless the person or being in question is a god, but the Gravemind isn't. IDK, I personally wouldn't describe the Gravemind as transsentient, just a creature that is almost at the state of omniscience, but I'm probably viewing the word to mean something it's not.

Omniscience is the state of knowing everything. The Graveminds most certainly do not. The Graveminds are transsentient because if you kill a Gravemind, you're only destroying the corporeal manifestation of it. As soon as the Flood creates another compound-intelligence, the Gravemind returns as if nothing happens. You encounter this first hand in Halo 3 when you kill the Gravemind by detonating High Charity, but it returns as if it was simply delayed, completely coherent with its memories intact. [i]"You think me defeated!?" "The Gravemind - it's trying to rebuild itself on this ring!"[/i]