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]