Yeth hounds, or simply yeth, were fearsome outsiders that hunted under the cover of night. They were intelligent if bestial creatures whose howls induced unbearable fright in those that heard them. 
The hideous yeth hounds resembled large greyhounds, standing 4‒5 ft (1.2‒1.5 m) high at the shoulder and weighing about 400 lb (180 kg). Their short fur was a dull, nonreflective night-black, often blending into the darkness and leaving only the cherry-red glow of their eyes and the smell of chilled smoke to indicate their presence. They possessed an oddly human-like visage, with protruding noses in place of muzzles and more flexible necks than those of canines. Their ears were short and pointed, curving upwards and away from the rest of the head in such a way that they resembled horns.
Yeth hounds were wicked hunters and fundamentally obedient creatures, following the toughest member of the pack with complete deference and consciously seeking evil masters to serve. Aside from that, they had no loyalty to each other and would not assist each other unless told otherwise. The only prize they sought from their prey were their terrified screams as they hunted them down and tore them limb from limb and any treasures found with a yeth hound were purely incidental.
Threats and intimidation were of no use against yeth hounds as they possessed only one crippling phobia Yeth hounds never went under sunlight even in the middle of a hunt and even if certain death awaited if they didn't. They made sure that there was always time to return to their dens before the strike of dawn and stayed there pacing or sleeping until night fell again. Not even the wills of their masters could persuade them to stay during the day.
The iconic feature of the yeth hounds was their dreadful baying that filled those nearby with panicked horror. Their ghastly howls filled foes with such unbridled terror that they would take the most direct path away from the yeth hound regardless of the obstacles or hazards that stood in their way. Their claws were not strong enough to cause serious damage but their wicked sharp fangs somehow assaulted foes mentally when they were in a terrified state. Evil outsiders were typically exempt from the effects of their barking but mortals could suffer permanent trauma from the supernaturally imposed fear. Yeth hounds were capable of magical flight, silently and tirelessly pursuing prey by gliding slightly above the ground. Their superb auditory and olfactory senses paired with their frightening speed made them incredibly dangerous hunters.
Only enchanted and silvered weapons could truly harm a yeth hound, as they partially passed through ordinary weapons as if they were mist. Their fear of sunlight was rooted in its detrimental effect on them, as exposure to daylight could banish a yeth hound to the Ethereal Plane. Torches and light magic could not harm the nightly creatures but so long as the sun shone on the spot they disappeared from they would be trapped in the Deep Ethereal. Once the sun set once more, the yeth hound could enter the Border Ethereal where it could see and be seen by beings on the Material Plane but not interacted with. Only by finding a pack mate or their master could a yeth hound return to the Material Plane, and death on the Ethereal Plane permanently killed them.
Yeth hounds exercised pack tactics similar to dogs but possessed greater intelligence. After hunting down their weary, fleeing prey from the air they would surround and rush the victim pulling them down and devouring them. Otherwise, their master determined their strategy.
Yeth hounds consumed warm-blooded creatures and had a preference for humans, demihumans and fey such as brownies, while they themselves were avoided by all natural and several unnatural predators. The nightly beasts did require food in order to survive but only once every lunar month.
Yeth hounds typically ran in packs of 4-16 individuals. These packs often made their dens underground in the depths of the most isolated of wilds. They could be encountered in Hades, but could also be found in any non-good plane such as Fury's Heart, Set's realm of Ankhwugaht in Heliopolis, or the wild plains of Spirac in the Abyss.
Yeth hound packs were often made by formidable fey such as the Queen of Air and Darkness and sinister forces from the lower planes as rewards for loyal servitors. It was theorized by several metaphysicians that yeth hounds were a manifestation of fear itself.
Yeth were known to establish telepathic connections with evil forces they judged worthy of being their masters, serving them as hunting hounds. The two could share senses and communicate from great distances while on the same plane, although barring this they could understand Common, Elvish, Infernal and Sylvan and responded to language comprehension spells despite being unable to speak in either case.
Yeth hounds typically seen to ally themselves with beings such as vampires, vile huntsmen, hags (although particularly night hags), wicked wizards like necromancers and other entities so long as they were immune to their terrible howling, as well as powerful members of the Unseelie Court. A master's death simply prompted them to seek new terrible forces to serve under.
I have heard the call of the Hound.
As in a nightmare, I run from that sound!
My bravery vanishes in fear’s flood.
Whence come the hounds?
In what cave do they lair?
Why have they chosen me, I who was brave?
What will become of me now, as fear’s slave?
Will I live until morning to breathe light air?
They will never leave, for they lodge in my dreams.
I know they’ll devour me, leaving only my screams.
After encountering yeth hounds, the warrior-poet Rodol of Ard retired from adventuring and never left his castle, banning all dogs and wolves from entering his domain. He lived for many years after the encounter and devoted himself to his poetry, never improving and suffering from nightmares each night before dying from a heart attack after hearing the howling of distant coyotes.
The yeth hound is based on the yeth hound of the folklore of Devon, a headless black dog known for its wailing.
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Achaierai • Barghest • Canomorph (Haraknin • Shadurakul) • Hell hound • Hordling • Howler • Larva • Maelephant • Marrashi • Night hag • Nightmare • Rakshasa • Succubus • Vargouille • Yeth hound
Fiendish creature • Half-fiend (Alu-fiend • Cambion • Draegloth • Durzagon) • Tiefling (Fey'ri • Maeluth • Tanarukk)