COMBINE

The Combine is a vast empire spanning multiple parallel universes. The empire is inhabited by an unknown number of sentient species, and appears to be governed by a race of bizarre, artificially evolved Advisors. The Combine expands its empire by invading worlds and enslaving the dominant species to be exploited as it sees fit. By manipulating these inhabitants through methods including bioengineering and implantation, the Combine creates a race of super-soldiers uniquely adapted for the environment of that particular world. This process results in a highly mobile and adaptive military force which is able to respond to any threat and crush any opposition. The reasons behind the Combine's imperialism remain unknown.
Source: Half-Life Universe, Combine Wiki