Being a noble on a horse with a sword doesn't make one a knight, unfortunately. The Byzantine cavalry also isn't quite the knight I'm looking for - anyone can be heavily armored and on a horse.
I think I'm going to use some of that, though, and also run with the ball from Eosin's idea, with some additional narrative on the classes for the general realm writeup.
So, for the "knights" what I have so far is this:
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A group of fanatics devoted to the ultimate development of tactics, originally comprised entirely (and still mostly) of Hobgoblins - often third or fourth sons not in line for any real inheritence.
In the beginning, such Hobgoblins would be fully equipped by their wealthy family and sent out into the world. Gradually, groups of these disenfranchised nobles created a loose association dedicated to sharing information and providing aid to one another.
As time went on, as is the nature of Hobgoblins, a strict heirarchy developed above the simple operative level. It was at this point that the organization began to accept contracts for services, much like a mercenary body. The knights themselves cared nothing for the cause, but if unusual battlefield tactics and highly trained warriors were required, then the right amount of money could get the job done.
Over the last few decades, the organization has rapidly expanded into neighboring realms, with Chapter Houses in most of the largest cities. As a result, races other than Hobgoblins are now finding their way into the organization, albeit in small numbers, so long as they wish to pursue the perfection of tactics above all else.
One side effect of this expansion, besides the recruitment of members of different races, has been the expansion of the original vision to include matters beyond the battlefield. The organization has always made use of spies and any number of "unorthodox methods" to accomplish their mission, but there is a growing realization that these endeavors themselves require a variety of tactics to succeed.
Thus now are spies and rogues, assassins and mages accepted into the ranks of the organization. A growing trend, though certainly not the dominant one, is for a group of individuals with widely different skillsets to join together into a special team that contracts out and works together. These teams are semi-autonomous, and able to handle a wide variety of jobs - the skills of each adding to the capabilities of the team as a whole. For large jobs, teams sometimes join together to accomplish a particularly difficult mission.
Each Chapter House reports to a regional council, which in turn reports to the organization head in Doluhre, Ulruz. The group is outside of any political stance, being concerned merely with the ultimate perfection of tactics in all its forms. To this end, teams and individuals are consistenly rated on their experience, performance and results. The resulting rating can mean a huge difference in terms of the individual or team one can hire for a given amount of money.
Recognizing that sometimes hirers and agents don't want to be known publically, the Chapter Houses act as blind agents. This means that the team or individual often will not know who hired them, and also that no one can trace a given mission to the hirer. Naturally, this is expedient to many governments and employers...
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Anyway, that's what I have so far. I'd appreciate comments, insults (jk) constructive criticism - I know my players won't be polite if there's a blatant hole...