THE PLAGUE
Classification: Imperium Morbus (controller disease)
Sapient? See text
The Plague are powerfully psychic microorganisms. Typically, they gather together and form colonies. A Plagueform - a single member of a Plague colony, similar in structure of a micro-cephalopod - has an incredibly high metabolism, and has a powerful acid inside of its membrane/shell structure. It is constantly consuming and channels almost all of its food into repairing the constantly damaged membranes. When a Plagueform dies, the acid inside of it rapidly consumes all internal structures, which, working together, completely neutralize it. It then leaks out of the outer shell and coats it entirely. These excretions, upon contact with air, crystallize into hard mineral deposits, typically forming in spherical or linear shapes. These pellets form, together, into a catacomb-like structure in which living Plagueforms make their home.
When a Plagueform dies, it leave behind a psychic "scar" localized to its corpse. When enough corpses gather together (usually enough to make the catacombs as large as a football field) the scars mutate into a fully self-aware intelligence linked to the catacombs themselves. This "hive mind" will exert direct control over the Plagueforms within its catacombs, and will recognize and refrain from attacking other catacombs and Plagueforms.
This is not the most astonishing ability of the Plague, however. A group of Plagueforms can invade an organism and consume it from the outside, and construct a new organic body around its skeleton called a Plaguemorph. Plagueforms controlled by a sapient catacombs will avoid consumption of natural weapons or useful devices and incorporate them into the Plaguemorphs. Some catacombs have taken to "remembering" the biological structure of their favorite Plaguemorphs and reproducing them with pure Plague biomass.