Imulsion is a low-viscosity, luminescent substance that is the driving factor behind the events of the original Gears of War trilogy. This highly unstable fluid has long been used as a fuel source by the human population of Sera, though it was not until towards the end of the Locust War that humanity became aware of its true nature: Imulsion is a living parasitic organism, capable of infecting and corrupting any and all living beings with an affliction referred to as Lambency.
The substance known as Imulsion was considered to be more of a scientific curiosity than a practical resource after its discovery, and was essentially useless until Dr. Helen Cooper created the Lightmass Process. The process was capable of refining Imulsion into an efficient energy source. For a planet hungry for energy and facing something of an energy crisis in the long term, Imulsion was the perfect solution: efficient, clean and fairly abundant. As Imulsion extraction and conversion began to pick up steam, a certain few nations reaped huge rewards from its sale, while other less fortunate ones were denied the profits. Over time, Imulsion speculation on the financial markets ran rampant and entire currencies and economies became linked to its value. This eventually resulted in a large economic collapse on the planet Sera when extremely low Imulsion prices crippled the financial sector. The economic crisis sparked military actions, and the few countries that possessed an abundance of Imulsion reserves soon found themselves at war with those nations who were not as fortunate, in a conflict known as the Pendulum Wars.
As the Pendulum Wars raged on the surface, Imulsion was also spurring conflict among the then-unknown subterranean race known as the Locust Horde. Caught in a brutal civil war, the Locust were pitted against the Lambent, which consisted of numerous creatures and Locust beings infected by the effects of long-term exposure to Imulsion. Facing the loss of their underground home, they elected to invade the surface to escape the Lambent, triggering their genocidal war against humanity mere weeks after the Coalition of Ordered Governments had claimed victory in the Pendulum Wars. Both sides used Imulsion extensively through the course of the conflict, both as a fuel source and in their weapons of mass destruction. Particularly noteworthy among the humans were the Lightmass Missile and the Lightmass Bomb.
In 14 A.E., the Lightmass Bomb was successfully deployed against the Locust Horde, killing millions within a large portion of the Outer Hollow by triggering the vast Imulsion deposits beneath the surface. However, it had the side effect of producing an aerosol form of Imulsion, which when breathed in for extended periods of time created a condition called Rustlung, which rapidly spread among humanity's survivors. By 15 A.E., Imulsion continued to evolve, producing much more violent mutations among Locust lifeforms, which humanity above remained largely unaware of.
By 15 A.E., during Operation: Hollow Storm, it was found that the Locust had become desperate in their civil war with the Lambent, and that the Locust capital of Nexus was under siege. Facing the loss of the Inner Hollows, the Locust elected to sink Jacinto Plateau, humanity's last refuge, in a last-ditch effort to stop the Lambent as well as humanity. However, the COG quickly caught on to this plan and turned it to their own advantage, evacuating Jacinto City before the Locust arrived and triggering the massive Imulsion reserves beneath the city by deploying the Hammer of Dawn against a Lambent Brumak, effectively destroying the city and flooding the Hollows. This destructive act killed most of the Locust Horde, but the Imulsion quickly polluted Sera's oceans and landscapes, giving rise to new and more aggressive Lambent lifeforms, among them the Lambent Stalks, Lambent Leviathans, and Polyps.
By 16 A.E., the Lambent began invading the surface, killing and infecting everything they came across, including humans. Eventually, heavily-exposed humans fell under the corruption of Imulsion, transforming into Formers; many Stranded who previously lived in Char and Mercy were such victims. In 17 A.E., Imulsion reached a critical stage in its life cycle, and the Lambent Pandemic was on the verge of infecting all life on Sera.
Professor Adam Fenix discovered Imulsion's parasitic nature and worked for twenty years to find a way to stop it. He created a superweapon that would destroy both Imulsion and anything sufficiently infected with it, which included the entire Locust population. Adam tried to find a way to save the Locust, but he had no time to refine the weapon as the Imulsion was on the brink of "a critical stage of its life cycle". To test the weapon, the professor forcibly injected himself with Imulsion, which began to rewrite his genetic code.
When Adam detonated his weapon, all traces of Imulsion were wiped from the face of Sera - obliterating the Lambent, the Locust, and himself. It appears that refined Lightmass Imulsion was not affected as COG-UIR aircraft and ships continued to function properly. However, the sudden global evaporation of raw Imulsion would soon trigger a new energy crisis for the hundreds of thousands of humans who had survived the Locust War and Lambent Pandemic. Over the next twenty-five years a technological dark age would define the civilization that was attempting to rebuild itself. Humans were forced to return to and consult traditional and alternative energy sources in the hopes of restoring the Seran society that had existed during the Era of Silence.
Besides its nature as a valuable energy source when properly processed, Imulsion also possessed a fascinating mutagenic effect. Direct exposure to Imulsion for any length of time could cause almost anything to transform into a Lambent form, making them highly aggressive, bio-luminescent and highly explosive when killed. In extreme cases, Imulsion exposure will cause the subject to mutate uncontrollably, such as the Lambent Brumak.
On the death of a mutated subject that does not result in explosion, the Imulsion was seen to escape the host body and move on its own.
Prolonged exposure to Imulsion, particularly in an aerosol form, was known to cause health problems in humans, chiefly the respiratory disorder called Rust Lung; Gold Rush miners had a shorter life span than the average human and many became infected.
As refined Lightmass, Imulsion was credited as being far cleaner burning and more efficient than traditional gasoline. When utilized in a generator, burned Lightmass would emit a yellowish exhaust.
Adam Fenix revealed that Imulsion was actually a living parasitic organism that mutated its host. He believed that Imulsion had properties of fungus and that the Lambent Stalks were actually Imulsion's fruiting bodies.