I'll probably have them have roots all the way back to the war that wrecked the world in the first place. Possibly something like a group of librarians, scientists, etc banding together to preserve their knowledge as society collapsed around them.
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Well I imagine Orglas has existed in some form or another since not long after the Calamity, since its position would be pretty prime real estate. Perhaps your faction (or some predecessor of it) has been around since nearly the beginning and aided in its founding. Or maybe they came around later and their knowledge helped the city rise to a state of greater prominence. Or something.
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Hmmm, do you have any idea how old you plan on your faction being? Since I can see some interaction between your faction and Orglas, and it's something for me to think about.