Nightmare (creature)

Nightmares, sometimes called hell horses or demon horses,  were fiendish equines named after the terrible dreams suffered by those that encountered them. They were wicked creatures best known for serving as mounts under a variety of malevolent beings throughout the Lower planes.

Description
Nightmares were seemingly emaciated stallions, approximately tall at the shoulders, with night-black coats. On further examination, however, their warhorse-like exterior was revealed to be entirely superficial. They had huge heads, fangs like vipers, and malevolent dark eyes often illuminated by red-hot flames, and they spouted orange fire when their nostrils flared. Wreathed in fire, their manes were wild and their tails unkempt, but they turned to cinders and quickly dispersed upon death.

Their steely hooves left scorch marks wherever they went but the tracks left mysterious patterns and gaps. They galloped through the night with unnatural agility, their black bodies blending into the darkness and leaving onlookers with only the sight of racing, infernal flames that appeared and disappeared in smokescreens of brimstone. Rumors had it that the creatures moved so fast that they could only be seen as a blur, but those who'd seen them speculated they could move between shadows or teleport, which would explain their ability to stop and start in an instant and suddenly appear without seeming to have moved.

Personality
Nightmares were callous and cruel creatures that stalked those they could victimize. They displayed hatred towards all life on the Material Plane and exhibited an infernal intelligence unseen in true beasts. Rather than simply murder their targets, they used their supernatural powers to strike terror in them first, making them fear for their lives and killing them once they were reduced to panicking wrecks. One of their favorite tricks was to allow their victims one last moment of feeling safe before suddenly reappearing, dashing their last remaining hope, and then striking the killing blow.

Despite being wild and roaming creatures, they were known to serve willingly as mounts during missions of malice, but only for those of sufficient wickedness and ability. Those that wished to ride them would be wise to remember that nightmares were not simple steeds but fiercely independent and untameable fiends with their own instincts and agendas. Prideful and egotistical, nightmares often acted upon their own wills even if such actions ran counter to the goals of their supposed masters and sometimes even at their expense. Even if both parties desired to commit evil deeds, their exact methodologies could be incompatible and attempts to force them into servitude often ended poorly. Only after forging a bond with them over time would a nightmare become loyal to a rider and only then would the two become true partners in villainy.

Abilities
The titular power of nightmares was their ability to appear in dreams, either before the victim had encountered them in the flesh in a kind of ill omen or in the minds of survivors after a terrifying attack. They could shift to the astral and ethereal planes, as well as the Shadowfell, at will, and despite their lack of wings they could magically fly even faster than they ran. Their hooves typically ignited flammable objects and they could leave massive walls of fire in their wake.

Nightmares were often surrounded by a haze of smoke thick enough to prevent others from seeing them but which they could see through perfectly. They often exhaled a searing, sulfurous smoke in the midst of combat, neighing in fits of rage while snorting out the noxious gas. The blinding, choking cloud left enemies caught within it wheezing and unable to properly fight back against the nightmare.

Combat
With tooth and hoof, nightmares savaged most creatures they came across. Beings from the lower planes were typically spared from their fury but other entities weren't so lucky, particularly those from the Material Plane. They utilized hit-and-run tactics, targeting separated members of groups and blocking off means of escape through fires, speed, and their ability to move between planes. Only skilled riders could properly fight with a nightmare while riding on top of it.

Society
Nightmares of the Shadowfell were unique in their tendency to hunt in small, predatory herds similar to wolf packs. They haunted untraveled roads, either in the Shadowfell or Material Plane, looking for the rare, night-going wanderers to prey upon. Passages known as "ember ways", so-called because of the smoking hoofprints left behind in the wake of their makers, were regarded as dangerous paths even by the denizens of the Shadowfell and even during the day.

The gloomy realms of the Gray Wastes and the bleak Shadowfell were most closely linked to nightmares, but they could be found and shaped by other planes—for example, the Abyss had mutated its nightmares into demonic variants. Such nightmares were usually lone wanderers that murdered individuals and smaller groups, but those with herds shared tighter bonds and possessed unique social structures not found elsewhere. Sin's Reward, a nightmare barded in spiked, black armor and cloaked in black smoke and white flames, was the master of all nightmares in the Nine Hells. Meanwhile the mares of the Abyss were commanded by the mutant mare Thunder of Hooves, a massive, six-legged nightmare that ruled through violence and intimidation. Despite their mutual hatred of one another, the two mated annually in the Shadowfell, producing three powerful foals: one would go to the Abyss, one to the Hells, and one would stay in the Shadowfell.

On rare occasions, somewhere between a year and a decade, a sinister night came for all beings of evil known as the Gloom Meet. The denizens of the Lower Planes, as well as the Shadowfell, met together in the Glooms of Hades to conduct dark diplomacy: settling their differences, creating new alliances, and discussing future agendas. Two weeks before the Gloom Meet began, nightmares heralded its coming, racing across the planes and becoming more noticeable throughout the multiverse, particularly in the Gray Wastes. In the final moments before the Meet, nightmares charged in a terrifying stampede towards their final destination, leaving a trail of hoofprint-shaped scorch marks behind them. It was by following them that a few planar scholars and adventurers found the Gloom Meet themselves, allowing infrequent accounts of such meetings to exist.

As Mounts
Nightmares were favored mounts among many malevolent races, to the point where there existed an Order of the Nightmare, a company named for their steeds. Night hags were known to ride them but they were primarily ridden by powerful and intelligent demons and devils, particularly narzugons, who treated them as trusted and valued companions. Certain undead like death knights, liches, vampires and even specters also took them as mounts. Mortal spellcasters, wicked warriors, and anti-paladins sought out nightmares for power and acclaim and the shadar-kai prized them as steeds. Their burning manes were not a problem for riders as they granted those atop them fire resistance.

Taming a nightmare typically required complete domination of the creature, with it and its potential rider dueling to determine its fate. Only when it was clear the only alternative was death would nightmares submit, but they didn't lose their individuality and wouldn't blindly follow commands. Mortals wishing to bind a nightmare into service could use a combination of the summon monster and wall of fog spells to bring them to the Material Plane. Appearing through the mist, the nightmare had to be fed an appropriate offering: 29 gp worth of platinum, ground into oat-like flakes that it consume. Whoever fed them would be their master for three days. Beings of the Lower Planes didn't need to do this, but whatever agreement they made was unknown to mortals. It was also possible to bind nightmares using infernal tack—typically reserved for narzugons—which allowed the wearer of the magical set of spurs to call the nightmare from across the planes.

Ecology
Nightmares had no actual connection to real horses and were only semi-material beings without the need to breathe, although they could become tired. They could easily carry hundreds of pounds and, so long as their was space on their backs, could carry any amount of undead. Up to of gold could be carried to and from the ethereal plane along with their riders.

Nightmares, even though incapable of speech, could communicate between each other through some form of empathy.

Diet
Just as they lacked the need to breathe, nightmares never required food although they typically consumed their slaughtered enemies. They had a preference for the meat of demihumans and lesser fiends, with manes, lemures and humans being particular favorites, but specific tastes could vary widely between individuals. Unless provided with such delicacies at least once each month they had a mild tendency to exhibit hostile behavior. If nightmares did need some form of nourishment, a difficult concept to prove due to their ability to digest inorganic matter, it was likely they derived sustenance simply by perpetrating evil acts.

Habitat
Nightmares could be found all across the evil-aligned planes from Acheron to Pandemonium. They could be seen racing throughout Carceri in the acidic jungles of Cathrys and blasting deserts of Minethys as well as in the fire and magma flows on Gehenna.

Subraces
Thought to be bred from their stronger kin were lesser nightmares, a variant equally despicable but more cautious then their more commonly known counterparts. While incapable of transitioning between dimensions at will they were still able to move between them with passengers in tow and had an impeccable sense of direction. They immediately attempted to escape good creatures that attempted to ride them but were excellent teammates for blackguards.

Cauchemars on the other hand were a particularly powerful and vile breed of nightmares that struck fear in even the bravest champions. Their smoke was even more overpowering, and they could carry over twice as much as regular nightmares.

Creation
Whether or not nightmares could truly, naturally reproduce was a point of contention, but a method existed for creating them. An agonizing ritual of dark magic that involved the removal of a pegasus's wings, resulted in its transformation into a nightmare and the abandonment of its noble qualities in favor of more despicable traits.

Death
Nightmares could inflict deaths so gruesome that revenge-obsessed phantoms could be created from the process, but unlike most fiends, could die of natural causes. Aging nightmares could have signs of their decrepitude in the form of gray hairs on their black coats or a hunched posture. When their time approached, nightmares made their way to the third layer of Hades, Pluton, to a spot called the Hill of Bone in order to die, normally bringing ruin to their former masters in the process.

The Hill of Bone was a massive graveyard, a mountainous heap of bones several feet tall and shaped like a giant equine skeleton when viewed from above. Permeated by an eerie silence, the only noises that could be heard were the occasional whinnies and chuffs, even when no living nightmares were present. The barely audible rattling of bones was a constant background noise and sometimes the area that formed the legs or head shifted as if to signify victory. Appearing from walls of mist, elderly nightmares occasionally cantered onto the Hill of Bone, saluting it with their heads held high. The Hill responded with its own welcome, an ear-splitting neigh that resounded up to a mile away and could drive listeners mad. When the echoes faded, the nightmare would immediately fall over dead and its skin would rot off alarmingly quickly.

The enigmatic happenings of the Hill of Bones all belied its true nature, as the means by which nightmares achieved immortality alongside their ancient ancestors. The removal of a single, scrap of bone was seen as a grave violation by the entirety of the nightmare race, prompting every member to hunt down the offender, return what was stolen and exact retribution on behalf of their forebears. The feeling of being watched on the Hill of Bone was thanks to the skulls, all of which possessed truesight and could still communicate, allowing them to give descriptions of graverobbers. The theft was reported to the next nightmare that came to die which would quickly circulate the information. If the nightmares failed to retrieve the remains, normally stopping after the death of three more of their kind in the hunt for it, the nightmare whose bone was stolen was doomed to permanently die and have its life force spread throughout Hades. Dying on the Hill was paramount for nightmares and some had it prearranged for their bodies to be moved there even if they died on other planes.

Rumors and Legends
The devil duke Alloces claimed to have created the original nightmares alongside numerous other bestial monsters and curses, but few scholars, wizardly or religious, outside of his own followers believed him.

Notable Nightmares
Sin's Reward and Thunder of Hooves were rumored to have been agents of Vecna sent to spy on and report back on the activities of devils and demons. Further speculations were that the two had been corrupted by their respective planes and that Vecna wanted both dead.