I could have sworn the caller in darkness was an older monster, but all my Google-fu reveals is that it dates back to 3.0’s Psionics Handbook, or possibly Dark Sun’s City by the Silt Sea. In all of these sources, a caller in darkness is made of dozens or even hundreds of souls who died in terror.
Pathfinder’s caller in darkness is a more run-of-the-mill undead, made from only a single soul (“a creature with psychic sensibility that died a violent death” says Bestiary 5). One important detail is the same, though: The caller in darkness eats minds, and by extension souls. So even a single caller in darkness is soon an amalgamation of many stolen minds, all trapped within the terrible swirling undead form. Facing such a creature is enough to make you despair…and if you don’t, the caller is happy to help you with its Wrap in Despair supernatural ability.
So between its nasty attacks (I don't want to face anything incorporeal that does 6d6 plus Wisdom damage on a simple melee attack), the usual hooks associated with intelligent undead (particularly ones with access to the minds of their victims), a tradition of mass deaths in their lore, and a killer name, there’s a lot for a GM to work with here.
Musty tomes mention that a rare artifact, a cloak of living skin with magical and perhaps even psychic properties, is guarded by “a caller in that dark place.” The adventurers’ researcher, a sage who specializes in subterranean races, thinks the passage may refer to a dark caller, a caste of dark folk of whom he has heard only rumors. The adventurers have no reason to doubt his surmise, as his advice for dealing with dark creepers and dark stalkers proved invaluable on a recent adventure. But translation is a tricky art, and the passage actually refers to a caller in darkness—an undead entity far more dangerous than even the priests of the dark folk.
Fifty years ago an unnatural sandstorm brought the desert to Tolemar…and with it, a plague of locusts and mindless mummies. Over time the sands have receded, but the abandoned city is still dangerous. Nor are mummies the only undead threat. The trauma of the initial sandstorm and the plagues that followed created a caller in darkness. The creature uses its ability to cause an aversion to keep interlopers away from Tolemar’s central acropolis. A magical spring there could restore water and drive away the taint of undeath from the city, were it to be unplugged.
In organized crime families, loyalty is everything. But criminal family ties often wind up ensnaring the members in ways they never expect. When one of the Miko sons tried to outmuscle a strange vishkanya, the golden-skinned man obliterated him without a thought—or rather, he obliterated the Miko boy with a thought, an excruciating psychic flensing. The trauma was so great that the young Miko’s consciousness persisted as a caller in darkness, which in its confusion fled back to the family home and soon consumed the minds of the rest of the clan. But absorbing so many members with such close ties and similar mindsets has had an odd effect on the psychic undead. Much of the time the caller in darkness is still a mind-consuming horror, but its lucid periods are frequent enough that it has managed to continue the Miko family’s vile schemes by passing orders on to (or just plain terrorizing) the right subordinates in a macabre parody of the clan’s usual business dealings.
—Pathfinder Bestiary 5 48
Trying to use this blizzard to catch up on some of my backlog. Hooray!