High Charity's Bane.
The Proto-Gravemind within High Charity is the shadow of a far greater threat, one that will unleash damnation upon the stars should it grow unchecked. High Charity's proto-Gravemind is a perverse mockery of a living thing, a gluttonous stump of Flood cells that remained behind when Halo's cleansing energies burned away the corrupted biomass which once supported the Gravemind.
A measure of Gravemind's intellect remains, though its new vessel must feed and spread in order for it to manifest fully once again. With every Banished soldier that succumbs to its embrace, the Gravemind's re-emergence grows closer. The mountain of half-dead flesh quivers in anticipation, and the echo of timeless laughter rises from somewhere deep below.
High Charity's Proto-Gravemind is a massive, highly adaptable biological weapon. It is a horrifying enemy, protected by every Flood beast on the Ark. Only fools and madmen dare to attack it without overwhelming force.