The Keepers of the Yellow Sign are cultists who have sacrificed their souls to Hastur, becoming his undead servants. They received their name in The Thrushmoor Terror, Strange Aeons expansion of the role-playing game Pathfinder, although its origin dates back to Robert William Chambers' short story The Yellow Sign.
They can pass as humans although they have an eerie aura and some telltale markings, such as an unhealthy pale exterior and a puffy face.
They hunt down unbelievers who have come close to the secrets of Hastur, such as seeing the Yellow Sign or possessing an artifact related to it. Most of them do so out of a desire to cause pain and misery, but will occasionally work for one of Hastur's clerics. The Keepers of the Yellow Sign are drawn to art, generally creations that are decadent and/or bizarre.
Keepers can drain the life force of others and, at times, become enraptured by the act of draining energy. While doing so may kill the victim, they also run the risk of being overwhelmed and destroyed by that life force. Keepers are known to have existed for centuries or millennia, staying on the fringes of society, therefore, most are unaware of it. They work in ghoulish fields (for example, cemetery security or grave digging) or pose as beggars or beggars, pretending to be crazy. They have some magical abilities like invisibility and the granting of curses.