The purpose of the Plague Ettin is to bring biological warfare to the Warp. Their alleged Shee creator, Dr Moreau, intended to send them, sneezing and coughing, into Albia to annihilate the populace, but they destroyed him and fell into space during the Albian volcanic eruption.
A gregarious nature, an attraction to all things unclean, and extremely prolific breeding habits make the Plague Ettins particularly bad associates for almost all other creatures. They are in fact unable to survive without bacterial infection, their digestive processes being disrupted, leading to malnutrition and death. Unlike Toxic Norns, they need a supply of normal food to survive.