Nightmare

Kvasir of Midgarth calls the nightmare the steed of the great undead, a wicked parody of a horse, dark as night and aflame with the Pit's fire, a bringer of death and a servant of darkness. Nightmares are ghostly mounts that can take solid earthly form but do not truly belong to this plane. They look like coal-black horses, lean and nearly skeletal, with fanged mouths and eyes like live coals. They are terrifyingly fast and fly without wings.

Where they come from is unclear. Their common names, Demon Steeds and Horses of Hell, hint at the Nether Regions as their likely home. They carry wraiths, spectres, powerful necromancers and vampire lords, and are rarely found without some such master of great evil. A nightmare bites and kicks and is far cleverer than any animal. Ordinary weapons cannot harm it, though magic, silver and magic weapons are said to work.

Nightmares hate all life and can smell a living creature a mile or more away; they are superb, tireless trackers. How they are nourished is unknown, but the usual guess is that the Pit's own energy sustains them. They dislike sunlight and moonlight and are strongest in the darkest night; in sunlight their powers are halved. Any nightmare can pass as an ordinary horse for up to twelve hours, but a mirror or other reflective surface shows its true shape, as can certain magics. Like a spirit, a nightmare senses invisible, ethereal and astral beings within 200 feet.