The Fetish of Eddermung is a sentient demon skull connected to a living demon corpse in the astral plane. The physical skull has been encased in a thin layer of living stone and, when placed atop a staff or spear, is used by the Eddermung tribe of hobgoblins as a watchdog. As hate incarnate, the skull both envies and despises all true life, cursing and demeaning them in every living language and ten thousand forgotten ones.
Superstitious, relatively unintelligent species (they must be sentient) can often be kept at bay simply by posting the Fetish outside a room or cave. It can see perfectly well in utter darkness or blinding light, and its half-phased body exists in the astral plane. This splice is what makes it immobile, but allows it to attack any beings that attempt to pass by it while phasing. More than one magic user has met his end by accidentally transporting himself while carrying the Fetish in his pack.
An adventuring party could make use of the Fetish as a sentry or a ward against wandering monsters reclaiming a dungeon that they are in the process of claiming. By carrying the Fetish in a sealed sack, or by securing its eyes with a blindfold, the Fetish can be transported in silence. If it catches sight of your party, however, its screaching could alert several hundred feet of dungeon to your presence.