The Combine Overworld is a location that appears to serve as the Combine's base of operations, from which they orchestrate invasions of other worlds - such as the invasion that subjugated Earth. It is likely the Combine's original homeworld, although it may simply be another world they conquered.
Very little is known about this world, as it has never been directly mentioned in any of the Half-Life games, except for a vague outburst by Arne Magnusson in the Episode Two chapter To The White Forest.
Through the portal seen at the end of Half-Life 2, an orange skyline of what appears to be many Citadels can be seen. As the Superportal was intended to reach the Combine Overworld, it is likely that the location seen was the Overworld itself. The two Combine Gunships attacking Gordon come from that location, hinting Synths are also built there.
The Combine Overworld's climate is heavily implied to be different from that of Earth. The Advisor for instance has a small black cup over its snout, suggesting it cannot breathe Earth's air. All Overwatch forces have completely airtight outfits, even though the wearers are humans on Earth; this suggests that although they do not need it on Earth, it could be useful in other locations such as worlds with a different atmosphere. The Overwatch Voice also tells several times to Overwatch troops that "mission failure will result in permanent off-world assignment", hinting any Overwatch unit is meant to be able to operate in other Combine-controlled worlds, the Overworld itself or other unknown Combine colonies. Before Breen attempts to escape through the portal at the end of Half-Life 2, he says to an Advisor that he cannot survive in the environment beyond the portal; the Advisor (presumably) answers he could do so with a new 'host body', which Breen hesitantly accepts.
 Behind the scenes
In the original final showdown conversation at Breen's office, Breen was to mention the "Aleph universe" as the universe to which Eli and Alyx were to be sent; this might be referring to the Combine Overworld. Interestingly, "Aleph" is the name of the main protagonist of Prospero, Valve's first game project, that the team probably recycled for it. "Aleph" is also the name of the first letter in the Hebrew, Arabic, and Persian alphabets, which makes sense as the homeworld is the "first" world. It is also a literary device used by Argentinian writer Jorge Luis Borges in his 1945 short story The Aleph to describe a point in space which contains all other points, and through which the entire universe can be seen.
- The Citadels seen through the portal are very similar to an early version of the City 17 Citadel used in cut propaganda posters.
- Like the retail Overwatch forces, several cut Combine enemies such as the Combine Assassin, the Alien Assassin and the Combine Guard use breathing masks, hinting they cannot breathe Earth's air, although this might be related to the old Half-Life 2 storyline when Earth's air was being replaced with toxic gas by the Air Exchange.
 List of appearances
- Half-Life 2 (First appearance) (Possible appearance)
- Half-Life 2: Episode Two (Mentioned only) (First identified as Combine Overworld)