In the beginning there was nothing but crap...... and well, Imulsion. Imulsion had to be the oldest living being on the planet and had survived countless eons underneath the crust of the earth. Imulsion was a sentient, living, breathing entity that could evolve and adapt. Imulsion was all fine and happy until science said, 'Let there be Life!' and KABLAM! You got freakin' non-Imulsion lifeforms. The Imulsion wasn't pleased and decided, "KILL EVERYONE!" and began noming and pew-pewing everyone.
With jokes aside, the Imulsion's plans were very... biological. It acted very much like a human's white blood cells... Imulsion viewed the Locust and Humans as a dangerous cancer seeing how they were destroying the world and themselves; acting as Sera's immune-system, the Imulsion began systematically purging the host's [Sera] infection from the inside-out. But Imulsion was far more cunning than one could ever possible comprehend! The Imulsion was harmless in its natural form, so it needed a host to interact with the world (like ghosts in some forms of fiction), so the Imulsion started to infect anything and everything that it could get a hold of. No matter where the Imulsion was, the endgame was the same.
Imulsion infected the Locust first because not only because most of the Imulsion was located deep underground, the Natives of Hollows were vulnerable to infection due to the years of processing it; Imulsion could also infect surviving-dwelling beings but it would take a certain amount of time to adapt to a certain genome. Imulsion was constantly adapting to suit its needs. If something was exposed to vast (but non-lethal) quantities of Imulsion in a short amount of time [the Lambent Brumak], it'd mutate uncontrollably beyond the fate of say, a slowly-corrupted Locust.
Obviously, Imulsion gave off lots of energy; believe it or not, the particles of a liquid actually move! True fact. The more an object moves, the more energy is given off. Things that give off lots of this energy tend to generate vast amounts of thermal heat, explaining the hot properties of liquid Imulsion. Suspended Imulsion was highly volatile because the energy wasn't able to contain itself, thus the only way to release that bottled energy was to explode. That is why, say, Lambent explode.
Adam Fenix stated that Imulsion was nearing a 'critical stage in its lifecycle' meaning the Swarm was reaching a godlike evolutionary level that would render Imulsion invulnerable and unconquerable - - if the Swarm won, it would've consumed every living creature on the planet and then return to the source, effectively curing Sera of the stain of life and returning peace to the planet. Over time, Imulsion would lose its genetic perfection through inbreeding (the lack of new cells and new DNA) and would eventually return to as it was. Life would've eventually find its way by then, allowing more sentient beings to arise, thus coming into full circle.
Luckily, Papa Fenix broke the cycle and saved all future lifeforms from death by golden goo.