This majorly contradicts my 'Imulsion is a Weapon' theory, but don't rule it out.
We all know Helen Cooper and the Lightmass Process in which Imulsion can be turned into an infinate, efficient energy source.
Another belief of mine is that Imulsion is an elixer of sorts. The Pendulum Wars have been raging across Sera for 100-200 years, I believe. This leads us back to the designer of the Lightmass Process, Dr. Helen Cooper. What if she took the Lightmass Process further than to create an infinate source of energy, just to gain the power of immortality. Know the creepy woman's voice in the trailers? This could be a 200 to 300 year old Dr. Helen Cooper, using the processed Imulsion as an Elixer of Life.
How can this link to the Locust Horde attack on E-Day? It could link in many different ways. Now, it must be a ***** to live underground, right? Why not use the Imulsion to keep the origional population alive forever, while they continue to breed, and so on and so forth. Yet, how could they have processed this Imulsion?
Remember the Forerunners in Halo? Ancient race, powerful, and which many believe were human before their death by The Flood. The Locust Horde could have been a much more intelligent, more powerful race in the past. This could explain their knowladge of weapons and battle tactics. Course, underground, they evolved with thick hide, night vision, and their ability to tunnel.
Thus they will be essential to Sera's past, directly linked to Imulsion, and to the future of the humans (well, duh). Brumak, Kryll, and Corpsers may have no part in this mystery, but you never know...