Combine

The Combine, also known as the Universal Union, is an immense multiversal multi-species empire in the Half-Life Universe, whose goal is to assimilate and enslave the multiverse.

When they invaded Earth in the early 2000s they were victorious in hours. Over twenty years they brutalized humans with drugs, mind-control and propaganda, and even acts of genocide to control them. They tried depleting the world's natural (especially oceans) and animal resources to the point of extinction. However, Some humans refuse to be subjugated and formed a Resistance that fought back against the invaders, beating the Combine twice, even with the powerful military they had at their command using their advanced technology.

They assimilate intelligent species from the universes they conquer, and re-engineer them into bio-mechanical organisms that serve as slaves. On Earth, the Combine uses their technology to create armies of super-soldiers called the Overwatch.

They teleport between universes and dimensions in their conquests, using teleporters that require dark fusion reactors to power them. Most of their technology is powered by Dark Energy.

Compare:

Borg