The Void Knights are a group drawn from the lesser Force Sensitives from within the Crusade, the reasoning going that if Force power per population is inversely exponential so that one person out of many (millions? Billions?) has enough power to be a Jedi or a Sith, then several people may have at least a large enough fraction of power as to be useful. As such, they come from all walks of life and have all manner of appearances before being abducted to join the Knights.
The long process of brutal mental manipulation and Force torture used to indoctrinate each Void knight as part of their nearly mindless collective gives them a hollow-eyed, sallow-cheeked look, although only the Maiden is actually blind. They wear black uniforms somewhere between a robe and a military outfit, and a large red star is coloured on the chest of their uniform to denote their position. They are equipped with lightsabers, which are gathered at cost to the Crusade and are thus valued greatly, sometimes more than the knights themselves. Most were picked up in the resale market spike that followed the ending of the official Jedi Order.
Mentally each Void Knight's personality is flattened and then subsequenly slaved to Dacian via the Force (he being far more gifted than any one of them). This is of course highly dangerous, for were they to collectively turn on him he could be overwhelmed by this Force bond. The Maiden keeps them in line, however, for her connection to Dacian is born mostly out of personal neurosies rather than outright Force control. This gives the Knights the semblance of a hive mind, near absolute fearlessness and the ability to relay orders to and from their master.
The downsides are many, of course. This process is dangerous (as mentioned) and the Void Knights, although less rare than true Jedi or Sith, are very difficult to replace. After the Xa'Fel situation it's most likely they'll be totally depleted and unable to rebuild their numbers from within the Crusade, requiring raiding parties to risk the outer galaxy to find new inductees. Not to mention that although more loyal, they are definetly less effective than a true Jedi or Sith, and indeed, in some ways are less effective than a fully functional human being.
That should cover it, I think. And yes, most are bald. Part of the torture.