The Unspeakable Possessors are a fictional race created by Chaosium in 1995 for the Call of Cthulhu RPG, whom were based on August Derleth's The Return of Hastur.
Unspeakable Possessors are a race created by Hastur, made from humans whom have made the Unspeakable Promise. All of whom eventually are possessed by Hastur and transformed. If the one who made the Unspeakable Promise is deceased, within a few days the nearest blood relative is possessed.
The victim's form remains humanoid but becomes bloated. It has boneless fluid limbs that end in tentacle-like jaw-tipped fingers and a gray-green skin with a scaly texture. The creature retains its intelligence and knowledge (e.g. the magical kind), but now is content in destroying, killing, and devouring.