Shorthand to Nells, these creatures (sometimes also called Nellies) are of the abyss. They live in dense vegetation, using the coloration of their bodies as camouflage to hide themselves amongst foliage. Their coats function well for this, as do their tails, which can fold over themselves when they rest on the ground. They have some control over the tail feathers' texture and coloration, and given some time can alter them to better mimic the area, for more effective ambushing. They are carnivores, eating any animals drawn in by the flowery dapples to their coat or to the 'safety' of the thicket they can make with their tails when they are in wait, but they have specifically evolved to hunt humanoids and other large animals.
The Nells have multiple features that assist them in their hunts. They have prehensile claws on their forelegs, which allow for grasping and tearing when dealing with meat. They have spurs for fighting one another, but also a defense against creatures approaching from an angle that they cannot reach easily with their beak. The beak itself is tough and thick with a sharp tip, used for rending meat and snapping bones, and inside their mouth are serrated teeth, which allow for grabbing on and keeping something from pulling free easily if it still lives. Even their tails can be effective, able to to flip as they do and knock prey to the ground. The feathers may also be 'condensed' by folding over one another into a thick tail-like structure, which can be used for balance while a Nell is running, or weaponized by swinging it with less resistance and more weight to the impact.
Their camouflage is still one of their greatest features, as it allows for ambush to be further effective, and their head, often colorful in a geode shape with 'gemstones' inside, often attracts creatures who care for shimmering objects or stones of value. Inside the stone is where they can release black tendrils of varying thickness, as they stay curled up inside the hollow until an animal is too close. They are prehensile, and can be used as a mechanism for subdual, and carry an electric current that can incapacitate.
The most devastating for humanity is their voice, however. Like mockingbirds, they can near perfectly emulate a call they have heard, down to the pitch. Often, they will hunt a person, and then use their own voice to call for help often with their panicked cries, to attract others to their location. Using bodies as bait, they will lie in wait in their self-made foliage, and ambush any human that wanders close enough. They are fast moving on all fours with such long limbs, and can fold their feathers together to make one long tail for balance if need be, but it often doesn't come to that. Instead, they conserve energy and wait for their prey to come to them, using familiar voices, or ones in need, to lure others to them.
When their prey is dead, they will use their beak to feed and tear a hole in the abdomen, which they will then use their electric tendrils to enter. The electric current they possess can bring false 'life' to corpses, which they can use to make them move and twitch; such movements can fool humans into believing the body is still alive.
While they cannot mimic a person they have not heard, if they manage to find a body that is fresh enough, using the same currents, they may be able to force the lungs and muscles to constrict in order to 'speak', which may allow them to learn a new voice they can utilize.
Nells all have the same gemstone on their face above their beak, which for a long time was assumed to be their eye, and a potential weak-point. It was quickly discovered that was not the case, and their eyes are actually what appear to be nostrils on either side of the eye (it is thought that they might breathe through the geode on their head instead). The piece itself is theorized to be used in mating rituals for Nells instead, as larger or unblemished stones seem to attract mates more effectively, paired with tail length and thickness.
Nells have some degree of sexual dimorphism, with female Nells appearing in blues, with ripples in their coats that are more effective to hiding in aquatic environments. Their 'geode' head appears more like coral or stone, and they keep it above water to breathe, and the gem they use to mate will often be blue and much smaller than a male's.
Once a year, the males and females meet at a mating grounds to have offspring, which are born with blue coats and stay with the mother until maturity. The males (and a few females) will have their coats change over time into more vegetative coloration, while most females and a few males will remain blue into adulthood.