The Space-Spawn are a race of aliens intent on conquering the galaxy. They are currently at war with Nokkar's people, the N'Kai. ("Sentinel") There is a third major race involved in this war, although in a mysterious and complicated manner, the details of which we do not yet know. Other races may also be involved to a lesser extent, but no more than twenty.
The Space-Spawn were formed "amidst the fury of an exploding star" in the distant past. They began to build an Empire for themselves, and quickly expanded in every direction, conquering new worlds. This put them in conflict with the N'Kai and a third major power, initiating a vast war between these three races, but nevertheless their expansion has continued largely unchecked.
On March 21, 2198, the Space-Spawn will finally reach Earth and quickly conquer it, abducting the human world leaders, the leader of New Olympus and the current heads of every gargoyle clan (as well as all the gargoyle eggs due to hatch that year) in a strike at Queen Florence Island. Simultaneously, they will steal the Master Matrix from Antarctica. They will then occupy and rule the planet, to be opposed by Samson and his Resistance. The Illuminati will enter into an alliance with the Space-Spawn out of self-interest.
Following the invasion of Earth, the original governments of the planet are still in place. (Greg Weisman has likened the occupation of the planet to the Vichy regime in France during World War 2.)
Very little is known about the Space-Spawn's appearance except that they only have one head.
The closest Earth equivalent to the government of the Space-Spawn is a militocracy. Nevertheless, the Space-Spawn are no more monolithic than any other species. Some individuals may disagree or object.
The Space-Spawn believe in two gods. Their legends claim that these gods existed before the Space-Spawn race and that they oppose each other, although it is unclear whether they have any existence beyond the legends.
Space-Spawn are highly technologically advanced. In part, this is due to a certain "bio-chemical affinity" that the species possesses for certain types of technology, due to their biology and anatomy. Other advances they acquired as spoils of war. They have an efficient and frightening ability to mobilize their war industry to support their war machine.
Some Space-Spawn may practice a form of alien sorcery. As Owen said "energy is energy," for the Space-Spawn: "power is power." 
A Space-Spawn would be able to survive in Earth's atmosphere without any form of magical or technological life support.
The Space-Spawn can travel naturally in space, having been born (forged) to do so. They also possess matter transporter technology and spaceships capable of faster-than-light travel. The transporters are inefficient in terms of energy, and they can only transport matter at the speed of light, which makes them impractical over great distances. Other forms of technology they possess include cloaking devices, force shields, memory-altering technology, and even weapons capable of destroying entire planets, which they developed centuries ago.
Although "Space-Spawn" is indeed the name they use to refer to themselves, it has also acquired pejorative connotations among the N'Kai (it is used as a curse word by Nokkar). The term arose, apparently, because this species was literally "spawned in space", formed during a supernova's explosion. It is unknown exactly when this occurred, although they are the youngest of the three races involved in the galactic war.