Every city attracts a wide variety of religious believers, from the established to the fringe. The Gatekeepers are a canonical example of the latter.
Using only heavy white caps as symbols of their fraternity, the Gatekeepers gather at various spots around the city which they’ve identified as “universal nexus points,” or “gates.” According to their texts, these gates connect this world to dangerous outer planes, but remain closed thanks to strange purification rituals that the Gatekeepers perform. Using an insanely complicated version of numerology, adherents identify new gates and abandon old ones every year or so, making them…difficult to ignore.
The Gatekeepers believe they are awaiting the arrival of the Key, an individual who will open the gates and fight an apocalyptic battle with the evil forces beyond them. The exact nature of the Key remains a hotly debated topic within the group.
As a friendly faction, the Gatekeepers are a harmless, goofy cult occasionally found on the odd street corner of any city. Their purification rituals involve silly walks, nonsensical chants, and burning unusual items like owlbear feathers and rare plants. Fortunately for the player-characters, this means the head of the local Gatekeepers–a doughty, pleasant, somewhat addled middle-aged woman named Rem–can keep a low-level party busy for months collecting unusual ritual components and negotiating with the owner of a building that contains a new gate.
As a foe faction, the Gatekeepers are a little more sinister. They insist on invading and “purifying” places they’ve identified as gates, and instead of burning feathers and spices they sometimes sacrifice small animals and offer human teeth, fingernails, and toenails. They often delve into dungeons beneath the city looking for gates, and will race against PCs to get to any treasure first (confiscating it for the Gatekeepers’ use, of course).
You can also up the anté by actually having the Key show up. Turns out, the Gatekeepers were right all along, and a naïve teenager appears who actually does have the power to open the gates, which in turn causes eldritch horrors to pour into the world. Whether the Key can actually fight them off is your call.