Since the time of the Nameless Empire, the Arisen have commanded devoted groups of mortals who serve them. Even during their long periods of sleep these organizations continue, passing along their secrets and duties to the next generation and watching over the tombs of their masters. Typically only one mummy oversees each cult, and if that mummy dies or disappears the cult may go rogue and use their eldritch knowledge for their own purposes. This might be the ultimate origins of Last Dynasty International.
In game terms, mummy cults are handled as a Merit with its dots split among several different aspects, like their tombs or vampiric havens or mage-enhanced items. The dots in a Cult Merit can be assigned to Reach (how widespread the cult's influence is), Grasp (how strong its influence is), and benefits (special aspects of a particular cult). It takes two dots in the base Merit to get a dot in either Reach or Grasp, but just one dot to gain a benefit. A non-mechanical aspect is the cult's origin: tribal, conspiracy, or enterprise. While the foundation only reflects a story element of the cult, it does determine the types of influence the cult might have and what benefits are appropriate for it.