Although little was revealed about the Knights of Byzantium, it is clear that they were inspired by medieval Christian military orders such as the Knights Templar.
ORIGIN: A well known Byzantine custom, later copied by the Romans, was to bestow upon each Royal Guard the title of “Protector.” The collective corps of these Protectors was called “Schola Protectores Domestici.”
Upon Constantine’s great victory against Maxentius in Rome in the year 312, [after he was proclaimed Emperor by his troops in York], when the miracle of the appearance of the CROSS occurred, the title of the Guards and Protectors was converted into the rank of “Chevaliers” who were the Guards of the “Royal Lavarum,” and the Order was later confirmed into the “Sacred Order” after Constantine’s victory at Chryssopolis in the year 324.
Hence the Byzantine stories are traditional in their plot structure and setting (featuring complex turns of events taking place in the ancient Mediterranean, complete with the ancient gods and beliefs) but are also medieval, clearly belonging to the era of the Crusades as they reflect customs and beliefs of that time
I don't think that the Knights of Byzantium are real, but they are very reminiscent of Crusaders. They were probably meant to show the point of "There's the pure evil and there's the evil that thinks it's right."