Grell were sightless aberrations that resembled giant floating brains with writhing tentacles and a large beak. Its central body was a large, pulsing brain-like organ, covered in a tough, rubbery hide that was nearly thick as leather armor. From this central mass extended ten long, rope-like tentacles, each lined with razor-sharp barbs capable of paralyzing prey. The grell's beak, hidden beneath its floating form, was used to tear into the flesh of its victims once they were incapacitated.
Grell internal anatomy was perhaps equally unusual and alien. Despite their appearance as a brain, they had an internal brain above the beak, and a stomach near the centre of the body. The creature maintained its internal structure via a soft and flexible cartilage cagework; a grell could compress its body to roughly half of its original height or width with ease, thus allowing it to easily squeeze through surprisingly small spaces. They lacked eyesight, but possessed excellent auditory abilities and skin that was sensitive to vibrations and electrical fields. This granted them a sort of electroreception that allowed them to navigate passages and sense other creatures.
Grell were ambush predators that had a strange alien intelligence, as well as their own language. They had no compunction about attacking any creatures they classified as edible, including humanoids, but tended to avoid bigger creatures that they had little hope of carrying away. Unlike many other subterranean creatures, grell often hunted in groups, using their collective intelligence to outmaneuver and overpower even the most cautious adventurers. However, there were individuals that lived separated from colonies, often picking and choosing their moments to hunt solitarily.
Grell colonies were found only in the Underdark, and typically numbered between four and twelve individuals, but could reach as high as fifty. This was not including any humanoid livestock kept to make feeding easier. Such unfortunates never survived for long, except for the few who treacherously led other humanoids into the clutches of the grell.