Denizens were a heavily engineered sentient species created on Europa by the Europan Demarchy. They possessed varying levels of intelligence, ranging from animalistic to baseline human capabilities, but were invariably of great physical strength and well adapted to life in the Europan ocean. Their most unusual adaptation was their ability to utilize hydrogen sulfide instead of oxygen in a modified human body -- Europa's oceans containing pockets of high sulfur content.
The Denizens were originally created for slave labour, but the project was abandoned after infighting amongst Demarchy researchers. A few Denizens, however, survived and retreated into the volcanic vents of Europa. Later on, Cholok, a Europan defector, tricked Agent Marius Vargovic of Gilgamesh Isis (an anti-Demarchy faction that controlled Ganymede and Callisto), into giving the Denizens a virus containing genes that would let them survive in the oceans outside the vents (to better control them, the Denizens were previously unable to survive in the cold oceans of Europa without assistance), at the cost of his own life.
The Denizens later mounted a rebellion, successfully destroying the first Europan Demarchy.