This is public information.
The Hermetic Order rules a small but important part of the city known as the Walled Quarter. Here, all the books on magic are concentrated and stored, and none but a member of the Hermetic Order may enter the Walled Quarter on pain of permanent exile from the city. Members of the Order regularly patrol the Quarter which, despite its name, has no walls save for a chalk border.
Members of the Order wear silver robes and heavy iron manacled chains, supposedly as a symbol of their dedicated service to the City. However, it is commonly believed that the chains are enchanted artefacts of some kind, used either as sources of magical power or by the Order’s masters to keep a leash on them.
This is privileged information.
As far as anyone can tell, there seems to be some sort of ranking system within the Order. Mid-ranking members wear plain blindfolds over their eyes (though they still seem to have no problems picking their way across the city), and higher-ranking members – in addition to the blindfold – start sporting intricate tattoos across their faces and bodies.