The Claustronoctiling originated from the belief of monsters that come out at night while people are sleeping. It resembles a terrifying and hairy bipedal creature with no feet and long claws. Its legs and antlers are made out of a mixture of lead and charcoal. Holes spot the Borderling's lower part of its legs. Below its ankles are sharp points. Even though it doesn't have any feet, it can stand on its legs perfectly.
The Claustronoctiling levitates 5 inches off the ground, but it can walk like it does have feet. The hairs on the Borderling's body are actually thin, feathery tentacles. Large tendrils make up its tail, up to 7 or 10 apiece, while thousands make up its fur. Smooth, red fluid drips freely from its fiery, glowing eyes. While its mouth is closed, the Borderling appears to have a small head with a long neck. But in reality, it has a huge, maw with irregularly jagged teeth. When it opens its mouth, it appears more terrifying,
A faintly glowing sack is attached to the Claustronoctiling's back, which feels similar to human skin. Inside the sack is its own pocket dimension entirely made up of infinite closed spaces.
There is no flesh inside the Claustronoctiling but black, odorless smoke.
The Borderling's tendrils can be easily pulled apart but will regenerate immediately. More solid body parts, like antlers and legs, can take up to 3 to 4 weeks to heal. A glowing violet symbol hovers above its head.
It doesn't vocalize; however, it makes low, guttural growls that only its own kind would understand. On occasions, they use hand gestures similar to sign language to communicate towards others of its own kind.
An impure Claustronoctiling will have dimmer eyes and a flickering, glitching symbol. Because of this, they are unpredictable and should be avoided at all costs. The Borderling's smoke is affected by the sickness, turning corrosive as it manipulates the victim they are punishing.