This knightly order consists of those who wish to explore and preserve Magic in all its many varied forms. Most are fighters originally, although priests and very rarely wizards could most definitely gain entrance into the Order of the Art (as the gods call it). It is not just worshipers of Mystra either, but her and all the other gods of magic associated with the Lady of Mysteries.
Latent Powers:
All of the Knights of Mystra are able to learn and cast spells, even in addition to other non-spellcasting classes. Thus, a fighter who becomes a Knight in the Order of the Art can become a multi-class wizard or priest (even more rarely, a single classed wizard or priest could with the DM's permission become a multi-class priest/mage).
This is not without penalty, however, as the character then must split his experience as a normal multi-class character (and gains all other benefits and hindrances of being multi-class), plus he or she must also earn double experience n the new class gained. For example, Hal Strikelin (seen below) became a mage in addition to already being a high level fighter, thus he must earn double experience to gain levels in his wizard class.
Manifested Powers:
Above the normal schools of magic a wizardly Knight of Mystra has access to, he or she has access to a school of magic noramlly not allowed to him or her. Thus a mage can have access to wild magic, or a necromancer could have access to illusion/phantasm spells!Likewise a priestly Knight of Mystra has major access to an additional sphere!
Furthermore, each of the Knights must adventure and created or gather the materials to have special armor or protective equipment for themselves that bears the symbol of Mystra and possibly Azuth or any other god or goddess under the Lady of Mysteries that they may worship. Although the symbol of Mystra must be at least as prominent on the armor as the latter god or goddess.
Only after this adventure is completed does the new Knight of Mystra gain the latent ability to become multi-classed and the above innate spell ability once per week (ten days).
Campaign Uses:
Nearly all of Mystra's Knights were very powerful individuals before becoming part of the Order of the Art, so care must be taken anytime such a powerfull character appears in a game campaign. Large-scale magical battles will certainly attract the attention of any Knights in the area, as will any potential cataclysm facing the Realms. Such events, even in the Realms, should occur only rarely. Much more realistic (and less damaging to a campaign's balance) is encountering one of the Knights in the course of his or her normal, everyday life. Such a meeting may be extremely important and exciting to PCs, but only routine for one of the Order of the Art.