In Babylon 5 the Shadows are an ancient species, older than the mysterious Vorlons. They and the Shadows are the last active First Ones, whose purpose in remaining in the galaxy and not going 'beyond the rim' was to guide the next generation of interstellar civilizations into peaceful coexistence.
They are large creatures, individually insect like. Their abdomens have six long, single-jointed legs, with sharp deadly barbs, while their thorax has two similar arms, with three long tine-like fingers, two of which are multi-jointed. Their heads are triangular and their thorax rises above their head giving them a sort of fringe. The appear almost like centaurs crossed with black widows. They have 16 eyes that are grouped in 4 clusters of 4 on their otherwise featureless face, the clusters are 4 eyes on a line which is angled up away from their vertical bisection. Each individual eye glows with a sort of inner orange glow, juxtaposed heavily with their black carapace. They are capable of making themselves invisible to human visible light wavelengths, though whether this is natural or artificial is unclear. They also give nearby telepaths extremely bad vibes, which is a serious condition for a telepath to be in.
The Shadows work in secret, they don't personally assassinate or manipulate politics, they convert others to their cause and have them act as proxies. This is where the character of Mr. Morden comes from on the show. They do the same thing with their own ships, they have telepaths from other civilizations imported to run their ships. The people being used are so intwined with running a star vessel that they are highly vulnerable to telepathic attack.
These vessels are black, two tiered semicircles with long spikes coming out of the top horizontally and on the bottom angled down, all of which are on the outside of the semi-circles. They move in the direction opposite the spikes. They are often referred to as Shadow Battlecrabs. The shadow ships are capable of launching small fighter craft and firing deadly beams of purple energy. They are also capable of phasing in and out of hyperspace, instead of using jumpgates or jump points.
They have a philosophy of rearing the next generation of civilizations which is a sort of trial by fire. They visciously attacked the younger races and made them attack each other, manipulated into war after war, for the simple reason that the most well evolved species in regard to space combat will be the result of their sculpting, much as they may have been in the distant past. This philosophy was incompatible with that of the Vorlons, the Vorlons favored order and stability. The Vorlons wish to impose order on the young races so that they will approach their eventual angelic space brethren state.
The result of this conflict of philosophies is war. The Shadows, breaking with the Vorlons, decide to wage war against the young races, mainly the Minbari who had the aid of the Vorlons and one time traveling semi-human. The Shadow war that occurred was devastating to the Minbari, but left the Shadows severely hampered, a great deal of their fleet had been destroyed. So they decided to sleep and rebuild for a thousand years, before doing the same thing again.
And it was for a thousand years that they slept, until the events of Babylon 5 began.