Imperium of Man
In name, the Imperium is ruled by the Emperor of Mankind, who founded the Imperium approximately 10,000 years earlier. In reality, the Emperor is barely alive, sustained by an enormous life-support system called the Golden Throne; a massive bureaucracy called the Administratum -- ruled by the High Lords of Terra -- makes all major decisions for the Imperium on his behalf.
The Imperium is, for the most part, a dystopian nightmare. Since the Emperor was injured, the Administratum has become a paranoid organization that seeks to hold the Imperium together in the face of numerous threats -- internal and external, real and imagined. The Imperium is essentially a huge police state, complete with witch hunts and an Inquisition.
The vast majority of the Imperium's citizens are humans, but there are "abhuman" minorities with unusual attributes acquired as they adapted to adverse conditions on colony worlds. "Ogryns", for instance, evolved superhuman size and strength to cope with high-gravity environments.
Psychic ability is an emerging phenomenon among the Imperium's population. "Psykers" have the ability to draw energy from an alternate universe known as the "Warp" and manipulate it for various effects. This is the same universe through which Imperial starships travel to achieve superluminal transportation, and the Warp is not uninhabited; it is populated with entities spawned by the thoughts and feelings of living beings in normal space, and the most powerful of these beings are extremely malevolent. Psyker activity can attract the attention of such entities and provide them with access to the material universe. The dangers of emerging psykers drive much of the Imperium's internal paranoia.
The Imperium controls near one million worlds scattered around the Milky Way. An organization of telepaths known as the Adeptus Astra Telepathica provides faster-than-light communications for the Imperium, which has massive military assets both in starships and combat personnel. Technology in the Imperium reached its height before the founding of the Imperium and slid backward considerably during a time called the Age of Strife. During the rise of the Imperium, while the Emperor actively governed it, the Imperium recovered much of the lost technology, but there has been little progress since, and technology is widely regarded with superstitious awe.
Because of the numerous threats it faces, the Imperium is in a constant state of military readiness, with thousands of warships and billions of soldiers ready to mobilize on short notice. The Imperium's special forces -- the Imperial Space Marines or Adeptus Astartes -- are even more mobile, ready to respond immediately to any military emergency and hold a planet if necessary until reinforcements arrive. Unfortunately, the threats to the Imperium are so numerous that the majority of the Imperium's military assets are actually committed most of the time to one war or another.
The Imperium would be a huge danger to any other government it encountered. It views all alien civilizations as threats (though on very rare occasions it will ally with them out of necessity), and it seeks to control all human colonies it discovers due to a fear of daemonic invasion instigated by uncontrolled human psychics emerging in the species.
The Imperium in Debates
The Imperium of Man has emerged into the versus debate community as a potential adversary for the Galactic Empire and other major powers in science fiction. Like the Galactic Empire, the Imperium has fleets of starships measuring many kilometers in length, a trans-galactic reach, high-speed interstellar communications, planet-killing superweapons, and a vast military-industrial complex. Quantification of the power of the Imperium is difficult, but it is widely considered a credible threat to the Galactic Empire (especially if its traditional enemies are not a factor in the contest).
- Codex Imperialis