Nightmare is the oldest character that I haven’t disowned yet, turning a mere four years old this June, with Mad following not too far behind. She was created as a crrAAAaaAAzzYY counterpart to a character that no longer exists, and ever since her unsightly origins I’ve been trying to mold her into a character I don’t despise as much as my idiotic childhood. I’m getting there, I guess. Maybe.

Adopted and raised by an owner that hardly knew of her existence, let alone the basics of pet care, the young Siberian Husky knew little past being chained to a backyard oak tree for the majority of her life. The owner’s son, often neglected and forgotten as well, felt kinship and pity for the dog, and thus tried his best to make both of their lives bearable. That is, until his dog contracted tuberculosis one day and stopped eating altogether. Knowing that something was wrong, the child tried doing everything in his power to keep her alive, but unsurprisingly, it was too little. Despite all of his efforts, pleas, and bargains, she succumbed to her illness and died. She was buried in the backyard and forgotten by all but the boy, whose guilt never left him, even in his last moments.
The first pangs of the apocalypse happened only a few years later. The boy was granted mind-altering powers upon the first shifts of the Earth, making him a rare breed of beings capable of bending reality. (Basically he’s a friggin’ wizard, let’s be honest here.) He quickly realized that people with his abilities are being hunted down and killed. In attempt to protect himself, he began to study the intricacies of his power more and believed that he could perform a ritual to separate part of his power into a different being altogether that would serve him and defend him from his foes. Though he was assured that this could theoretically be done, he soon found that he would need a vessel body to perform the ritual on, and he knew the perfect host. Thus, his dog was reborn as a creature existing solely on the mental plain. She was now able to jump from mind to mind and alter thoughts subconsciously as the subject is awake and even appear and communicate with the subject in its dreams.
At first, she was only used to gather intelligence on his enemies, but she found herself becoming fascinated by the way her presence caused fear and paranoia within her hosts. Partially due to rising danger and otherwise due to his companion’s request, the boy began to go on the offensive, ordering his creation to incapacitate and even kill his enemies. It wasn’t long after all of the boy’s threats were eliminated that he began to realize that his creation was still preying upon humans, but all victims were completely innocent. One night, he confronted her about this and even threatened to take away her power, thus reverting her back to a corpse. Angered by the threat, she killed her master, stole his power, and rampaged across human settlements. With no chains to bind her, there were few limits to the terror she caused. Townspeople quickly caught wind of the phenomena and dubbed the horrific creature invading their dreams “The Nightmare”, a title and name she soon donned with pride.

Nightmare, as she is now, does not exist in the physical world. She can only be heard and seen by whoever’s head she is currently inhabiting, and she can travel from subconscious to subconscious at will through a plane of existence she believes only she knows of. Certain creatures can hear her voice talking to them as they are awake, but most humans will have to be heavily mentally incapacitated or asleep to be able to even know if Nightmare is there. While her influence on the brain in incredibly minimal while the subject is awake, she has full control over the host’s dreams and considers the psychological torture and eventual murder of her hosts to be somewhat of an art form.
From the dreamer’s point of view, it appears that the dream’s subjects and setting are manipulated by a cloud of black smoke that seeps around every corner. This smoke is an extension of Nightmare’s body and can be condensed into a liquid and solid if need be. Each state of matter the darkness takes has a different effect on the dreamer. Taste them all!
Nightmare is a large fan of so-called cinematic fear tactics and believes that presentation is much more important than getting a job done. Therefore, she has manipulated her primary form to look more human, as she has noted that subtle humanlike qualities added to her dog form are more likely to scare her hosts. This includes hair running down from her head to her back and finger-like paws. The red markings allude to her bloody death and the Y on her head was a symbol used by her deceased master. Some things just cannot be permanently changed, she supposes.
Nightmare can also dissolve and reappear through a cloud of smoke. On occasion, she may present herself in her original dog form, but this is rare and is usually used because she believes that the psychological effect would be greater in some way. She likes chocolate for the irony of it.

Brilliant stuff.
Probably pretty unsatisfying feedback, considering how much time and effort was required to draw and write up all of this (as well as thinking ahead of time about what to write and how to organize it), but it's the shortest and sweetest way I can possibly condense my criticism, or lack thereof.
I bet this thing also has a knack for some of that classic sleep paralysis.