The Shadows are one of the First One races of Babylon 5. They evolved millions of years before Humans and the other Young Races, and they regard the Young Races as children needing their guidance (which means "tough love", in the case of the Shadows).
Shadows appear to be large arthropods, standing approximately two meters tall. They have six legs for walking and two arms for manipulation. Their carapaces are black, and they have numerous glowing, orange eyes on their heads.
Shadows can make themselves invisible to normal light at will; whether this is a natural ability or a technological trick isn't known. They may also be able to make themselves intangible to some degree, as they seem to be able to easily walk through crowded corridors without bumping into anyone. They can not hide their presence from telepaths, however, who find the nearby presence of Shadows extremely disturbing and can at least dimly see them. Video cameras set to receive certain non-visible light frequencies can also see Shadows.
Intangible or not, Shadows can be affected by fire from the energy weapons of the Centauri, and presumably those of other races. A burst of fully-automatic fire from a Centauri small-arm is sufficient to disable or kill a Shadow.
What psionic abilities, if any, that the Shadows possess are not known. They are known to communicate verbally with their human agents via high-pitched chirping sounds. They apparently have no difficulty understanding human speech.
How the Shadows govern themselves is unknown. They do not actively involve themselves in galactic affairs most of the time, but approximately every 1000 years they instigate conflicts around the galaxy to stimulate evolutionary improvements among the Young Races. Their philosophy is that their charges must face adversity in order to advance. This philosophy is opposite that of the Vorlons, resulting in an ideological conflict between the two races.
Their base of operations in the Milky Way is the planet Zha'Ha'Dum, which lies near the "rim" of the hyperspace beacon network that allows the Young Races to navigate. The planet is protected by a technological or psionic guardian that controls the minds of intruders into the system, forcing them to land on the planet and surrender themselves to the Shadows. In case of invasion by their enemies, the entire planet has been rigged to explode.
The Shadows have a number of servant races -- including the Drahk -- who assist them in their endeavors.
The Shadows are advanced users of organic technology. Their starships are living things grown to perform their function. Despite their organic origins, Shadow warships are far more powerful and durable than the inorganic ships of the Young Races. The beam weapon of a typical Shadow warship can cut a large warship of one of the Young Races in half in less than a second, indicating that they carry hundreds or thousands of terawatts of firepower.
The ships are not, however, intelligent; each one has a sapient being imprisoned within, providing the ship with an intelligent pilot. The need for an intelligent operator leaves the ships vulnerable, however, as the pilot is susceptible to telepathic attack. In the last Shadow War, the Shadows attempted to reduce this vulnerability by acquiring telepaths for use as pilots.
The Shadows are one of the most powerful forces in the Babylon 5 setting. They have no interest in territory or conquest, but they will freely attack any race that seems too complacent in their effort to force races to evolve by surviving conflicts. They do not distinguish between military forces and civilians in their attacks. If their plans are sufficiently threatened, they can deploy weapons of mass destruction capable of rendering a planet uninhabitable in a matter of hours.
Shadows in Debates
As one of the most advanced and powerful races in Babylon 5, the Shadows are frequently proposed as opponents for powerful civilizations from other science fiction franchises in versus debates. Fivers consider them to be capable of destroying virtually any opposition, but evidence from the series indicates that while they could potentially overwhelm the Federation, they would pose little threat to the Galactic Empire.