In the half life series who are the combine

The Combine are an advanced alien race from another universe that take over other planets. They found Earth when they took the 'tiny fracture' at Black Mesa and forced it ever wider, pouring through in greater and greater numbers. War followed, and humanity lasted seven hours. Doctor Breen, speaking for all of Earth, surrendered to the Combine. They rewarded him with power. I can only imagine why Breen refers to them as Earth's "benefactors."
The Combine's soldiers are captured humans who have been mutated into soldiers (We see a soldier without his armor on a security feed in the chapter Nova Prospekt). The striders, hunters, gunships and drop-ships are biological mechs whom the Combine built to keep the humans under control. As of right now, we really don't who the actual Combine are. One of them would be the Advisors, flying grub-like creatures with telepathic powers who suck who-knows-what out of the backs of human heads using a proboscis.

We get a glimpse of the Combine Over-world through a portal at the top of the Citadel. There is an orange, cloudy sky and dozens of what I believe to be citadels.