Yarghuul
Fallen Sky. In the tales of old is a peculiar legend that speaks of a night when the sky was alight, basked in strange greens and greys colours. Then, tears from the heavens showered the world, comets, and meteors of various sizes. Strangely few of them were found, most dissolved in a matter of hours or sunk deep beneath the earth, leaving barely any trace of their existence. However, soon after the land became sick, grey, colourless and mutated. Beings emerged out of nodules of the remaining meteors, pouring out like white slime before taking the shape of squat, pale, toadlike creatures, superhumanly strong and noxious-smelling, having no eyes but instead a mass of tentacles at the end of their snout.
Toxic Brood. Yarghuul are aberrations that transform their surroundings into toxic wasteland, choked with patches of creeping and gibbering mounds of organic matter. They thrive in the desolation they create and bask in its strange glow. As such, they are rarely found in healthy lands and their presence means that a region is contaminated. They are very territorial predators, attacking on sight any living thing. They are often found in broods of five to ten creatures, but much larger broods have been discovered as well, truly infesting nightmarish regions of the deep Underdark. Their social hierarchy is nebulous at best, as they almost never communicate with other creatures, but they seem to share a hive-mind mentality.
Mother of the Brood. A Yarghuul infestation usually follows a simple scenario: they gather in a small brood and soil the area over time by their mere presence. Once their surrounding has mutated to their liking, they establish a nest. The brood scavenges any kind of materials and organic life they can and shape a large “shell” from their findings. Once the shell is completed and sealed, half of the brood will liquefy and infiltrate it, merging inside the shell. At that moment, the brood is the most vulnerable, its numbers depleted. Soon after, the shell will start emitting blinding light and deadly energy, vaporizing any that come too close. About a week later, a single yarghuul brood mother emerges from the shell, ready to lead the brood.
Telepathic Defiler. Yarghuul are driven mainly by instincts and are savage and direct in a  fight: they rush, devour and secrete toxic slime through the large pores of their skin. However, they are no mere beasts and work with other members of the brood to corner prey and identify the greatest threat. If mounting an ambush, they will use their mutable form to devastating effect, falling from the ceiling or emerging from the ground to surprise their victim. Members of a yarghuul brood use a complex combination of telepathic images, whale-like singing and bioluminescent pulses from their tentacles to communicate with each other.