The Grool is the main villain in the thirtieth Goosebumps book, It Came from Beneath the Sink!
This horrid creature is frightening yet interesting. It is composed of spongy epidermis similar to that of a common household kitchen sponge. The Grool's texture, however, is much slimier as it produces bile through microscopic ducts controlled by a narrow but complex central nervous system.
This intriguing system varies from us humans in the sense that the Grool has neither a way nor cause for reproduction, digestion, or respiration. The tiny nerve synapses are kept far from each other in hopes that the creature will stay focused and attached to its original owner. This strange phenomenon is brought about through the oddities ability to send unbelievably powerful electric impulses outward, which in turn can confine a single person to a life or death symbiosis with the Grool.
This mutually assured destruction means that, if the Grool becomes separated from the owner at such a distance whereas the impulses are severed, its captive will die due to all loss of electrode brain function.
The Grool strangely feeds off misfortune which can actually be explained by the same nerve synapses aforementioned. These synapses collect data as energy basically like a photosynthetic computer and the conversion process occurs instantaneously.
The Grool's only weakness is collecting data interpreted as pleasant, as this causes an overload in production of serotonin, a toxic neurotransmitter which causes the beast to regress until it is as harmless as an average dish-washing sponge.
Grools basically dwell in damp caves deep within rainforests and some have even been spotted washed ashore by the tides of the Galapagos, of course causing havoc to those islands. Though they seem easily defeated, their ominous presence and abilities causes fear and anguish to be amplified by twenty three percent, reducing chances of defeat the more powerful it gets.