Shimut the Flesheater, a member of the loosely defined "Heresiarchy of the Twelve" is said to be the sorcerer who invented the concept of the Shadow Sword. As with literary character Beorn the skin-changer from The Hobbit, Shadow Swords are "under no enchantment but [their] own." Able to access a small amount of magic--or The Shadow, as it is sometimes colloquially called--Shadow Swords can do basically one thing. Other than that, their abilities are exceptional, but not supernatural.
Shadow Swords are more common among the Northlander race, although even there it is rare. Very few others have learned to harness the Shadow in the manner in which Shadow Swords do. Northlander Shadow Swords usually belong to one of the many strange Death Cults from the Cannibal Island and elsewhere which produce mystical assassins, of which the Shadow Sword is the most prominent variety
Some hamazin kemling revanchist cells among the Cherskii Mafia have also learned the procedure from some unknown vector, and as the Cherskii Mafia has lost its original focus, the knowledge has slowly seeped out to a few others. Mostly still limited to assassins and other purveyors of "black ops", most people in the Land of Three Empires still have no idea what a Shadow Sword is or what he can do, and those who know the skills of a Shadow Sword are very discrete, handing down their knowledge rarely and carefully to hand-picked apprentices only.
The most iconic ability of Shadow Swords is the ability to manifest a weapon of pure Shadow at will. Despite the name I've used to represent the class, this weapon can actually be any weapon that the character desires (although curved sabers seem to be among the most popular). This includes missile weapons, even--although not any complicated mechanical weapon, such as a firearm. Any melee weapon, any thrown weapon, and any missile weapon other than firearms can be manifested as desired by the character, summoned at a moment's notice, and then "banished" again back into the Shadow as desired.
This weapon is usually a cool, matte black in appearance, and often seems to seep or exude darkness almost like smoke. Nobody but the Shadow Sword himself can use this weapon (i.e., he can't hand it to anyone else, and nobody else can pick it up if it's thrown, for instance--usually if it's thrown, the Shadow Sword would immediately disperse it and resummon it back into his hand again as soon as it has done its damage.)
Because a Shadow Sword is always armed, even if he appears not to be, those with this ability have naturally gravitated towards "black ops" type professions and assassination, to which this ability is remarkably well suited, but this is a trend not an obligation to those who belong to this class, naturally.
Shadow Blade: Gains combat bonus advantages to attack and damage, but only when using a
shadow sword; a blade of pure shadow substance that can be summoned at a moment's notice. Also, any unarmored character with this ability can add ½ his character level (rounded down) to AC, and can, instead of attacking, deflect missed missile attacks, including bullets, back at anyone shooting at him (rolls as if making a missile attack himself, can only apply to missed shots; hits still do damage as normal to the Shadow Sword character and cannot be deflected.) If the character doesn't move or take any other action, he can use his shadow blade to “fight defensively") by adding +4 to his AC against missile attacks. Missed attacks when “fighting defensively” can be deflected.
Shadow Swords also can cloak themselves in Shadow, which makes them difficult to spot. They gain a class bonus of +3 to Subterfuge. (I know, I know; Subterfuge is also used for a number of other actions besides literally sneaking around, including bluffing, creating a forgery or disguise, lying, etc. That's OK.)
Some designers' notes, briefly: The class started off being, basically, a Jedi from my Star Wars game. I decided to ditch the Force powers in favor of making him sneakier (as well as looking less obviously like a Jedi), so it ends up feeling, in many respects, more like a hybrid of the Assassin prestige class and the the Soulknife, rendered in the much more simplistic m20 ruleset. I long ago had the concept of an Advanced Class version of the Soulknife as part of my d20 Modern version of the setting, although it was an esoteric option that wasn't always easy to find. As I said earlier, I've often been back and forth on the fence about whether or not I think it fits my conception of the setting. Currently, I'm leaning much more towards making the setting a kind of swashbuckling noir setting rather than the somewhat darker version of the setting which is more Lovecraftian horror in sword & sorcery drag. The class fits better in the former tone than in the latter, and I think my drift in that direction has been a long time coming and is probably permanent, so it fits as I see the setting now, and probably will do so for good.