The Keepers of the Yellow Sign are cultists who've sacrificed their souls to Hastur, becoming its undead servants. Whom hunt down unbelievers who have gotten to close to Hastur's secrets, such as seeing the Yellow Sign or possessing an artifact. Most of them do so out of a desire to cause pain and misery, but occasionally they will work for one of Hastur's clerics. The Keepers of the Yellow Sign are attracted to art, usually creations that are decadent and/or weird. They are able to pass off as humans though they have a disturbing aura and some telltale marks such as an unhealthy pale exterior and a swollen face.
Keepers are able to drain the lifeforce of others, and sometimes become enraptured by the act of draining energy. While doing so can kill the victim, they also risk being overwhelmed and destroyed themselves by that lifeforce. Keepers have been known to exist for centuries or millennia, keeping to the fringes of society, thereby remaining unknown to most. They work in macabre fields (e.g. graveyard security or grave digging) or poses as mendicants or beggars, pretending to be insane. They have some magical abilities such as invisibility and bestowing curses.