Nightmares have many nicknames, demon eyes, night devil, hallowed chest, Light Eater

A Nightmare is called a 'Light Eater' because when the Nightmare syndrome awakens Within a Dreamer it slowly start eating the Dreamer heart reflection in there chest and replaces it with itself.

A Dreamer heart reflection is part of the Dreamer Light magic and who they are as a person, it core is pure light but the outer layer will have a color to it of what ever the Dreamer Magic color is.

when a Dreamer is infected with Darkness the Nightmare syndrome awakens and begin to eat the Dreamer heart as it's doing this the Nightmare is copying the Dreamer memories, and learning as it grows, if left untreated the Nightmare will completely devour the Dreamer and replace it with itself in the Dreamer body, the Dreamer body is referred to as a 'frame' and the Dreamer they devoured they referred to as "their former self'
For the most part it is believed what a Nightmare really looks like is the black and shadowy liquid, which is true in some way, it is what they look like Inside a Dreamer body and what they look like when they died, but a Nightmare has an actual form outside a Dreamer body thought most don't even know what they really look like.

There are a few individuals Soledormir, Void,the voice, Killy, Omen, Ai, Malice (there might be a few more that can to...) that know how or can remove a Nightmare from it's Frame, when removing a Nightmare out of it's Frame is called 'rip or ripping' it's not a pleasant experience for the Nightmare, the first time is the worst the Nightmare usually undergoes a type of shock, they have to get use to the sounds, feels and senses of there own body,
Soledormir design Nightmares to infect a Dreamer body and corrupted it, it's unnatural for a Nightmare to be out of it's Frame, they become extremely weak and vulnerable, but despite their frail body's a Light Eater senses increase, there hearing, smell, eyesight, and Predator sense.

The behavior and diet changes as a Nightmare establishes itself, they go from eating the Dreamer Light to absorbing negative emotions with the corrupted Dreamer Frame and use that for energy.

With out a Dreamer body a Nightmare can not absorb and process negative emotions and trun it into energy, that they need to live, out side there Frame a Nightmare average lifespan is 24 hours before starving to death, they can however live longer if there able to eat light Magic or things like it.

(This is why Nightmare like the taste of Fairfin because they are creatures of Light magic.)

The weather climate can decrease the time a Nightmare has outside there body to.

A Nightmare main instinct outside their frame is self preposition, finding food and surviving

Malice can rip a another Nightmare from it's Frame but the process kills the individual, he then use what left and put it's in his healing pool for a type of spell to Quicken healing and help with echoes.

Guaranteed Mal has been ripped out of his frame more than any other Nightmare has, mainly because of Omen, ether it's the only way to get Mal to do something or Force him to calm down or Omen just want to play one of his favorite games with Mal called ' hide the body' it's where Omen will rip a Nightmare from there Frame and hide there body and they have to find it before they starve to death or get killed looking for there Frame ( Omen Never let Mal get to that point, if Mal can't find his body in time and pass out, Omen puts Mal back in his Frame) Omen has almost killed Mal by ripping him out of his Frame, mainly when he's in his tormented form, it's a lot harder to rip a Nightmare out and they always resist and make it worse on themselves.

Omen has always put Mal back together and keep an eye on him, Mal actually would not have as much as an personality if it wasn't for Omen and his antex.

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Depending on the size of the form a Nightmare in when they get ripped out will change the size of there Light Eater form, but for the most part whatever form they are in they will be a quarter or less of that size.
Pyro Nightmares are darker in color and warm to the touch, they can't handle colder claimants very well outside there frame.

Frost Nightmare are lighter in color, and can have splashes of grey and white on them, there Paws are usually bigger then Pyro's, they can't handle warmer climates very well.

Both pyro and frost have 4 eyes, one they see out just like regular eyes ( only with more details) and there second is with their predator eyes, everything is seen though an aura and work similar to X-ray vision.

Not all Light Eater have Wings, even the one that do usually don't use them much because they take up a lot of energy.

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What the Horns and markings look like depends on the individual.

Light Eater also give off a Pleasant smell, ( it's bad enough there cute but they smell good to )

Light Eater have 6 legs, each paw has four toes including the thumb, there second pair of forearms they use to pick up things with.

Light Eater have several rows of sharp little teeth in there mouth, the Inside of there mouth glow as well.

When been ripped out, a Nightmare usually resist ( and just make it harder on themselves) the dark Shadow -liquid will gather at the stores that is ripping them out and ether die from shock or talk there true for of a light Eater.

The Nightmare frame becomes lifeless without the Nightmare Inside to controlla body, depending on how much the corruption has tainted the Dreamer body and how long the Nightmare has been outside of it, the Frame can start to show more Dreamer features again but will revert back one the Nightmare has returned.

Returning back to it's Frame, the Nightmare will use the Shadow streamers a long it's back, they will revert back into there liquid form and absorb into there Frame body again

I think that all I feel like i miss something :T

That is so awesome, you've developed them so well!! And they are too adorable for their own good. XD

Awwww sooooo cute