The hellhound is a generic name given to large black dogs primarily found in western Europe. The dog is over sized, black and has fiery red eyes. They are typically regarded as portents of death, and may be invisible to all but whom the dog hunts. Many were also guardians to the underworld or hell according to legend.
There are many regional names for the black dog, particularly in the British Isles. These may include the Barghest from northern England, the Black Shuck in Norfolk, the Dip according to Catalan mythology, or Cŵn Annwn in Welsh mythology and folklore. Large black dogs associated with death are known across many other parts of the world, particularly Latin America. However these are likely separate creatures from the traditional Hellhound.
Hellhounds have incredible strength and, despite their large size, are very fast. They exhibit phantom-like characteristics and may be visible only to the person they hunt. Otherwise the only distinguishable trait that may identify the presence of a hellhound is a distinct sulfurous odor. They cannot cross running water.
Hellhounds are manifestations of the Pattern that take the shape of a dog. They arise upon great need to restore balance to the Pattern. Should the Pattern sense that an individual thread (or soul) will disrupt the flow of balance in the future, a bubble emerges that hunts the thread until the individual is killed. In doing so, the thread is woven out of active life in the pattern and balance may be restored. This explains the hellhound's relentless hunt of an individual and seeming disinterest in all others. They have only one target and one purpose at a time. Once they have served their purpose, they cease to exist.
While within the present world, hellhounds are in the shape of the large black dogs of myth. They may be killed but this is likely a short-term solution, as a new hellhound will emerge to continue hunting the original target. Therefore the only reliable way to rid oneself of a hellhound is to 1) kill their target for them, or 2) significantly alter the thread involved so that they will naturally return to balance within the Pattern without the need to kill them. Note: the latter is almost impossible to do as the future is impossible to foresee.
However, there are individuals (prophets) that can glimpse the future. Prophecies of this nature work because the individual has a supernatural ability to "read" the Pattern as it is (and will be) woven. Prophets of this kind are the only non-targets capable of seeing a Hellhound.
Prophets that can glimpse ta'veren (vortices that warp the Pattern around them) can also glimpse Hellhounds and are likely capable of knowing the reason a hellhound has targeted a certain individual.
Age of Legends
During the War of Power in the future 2nd Age (the Age of Legends) a Forsaken by the name of Aginor discovered the nature and purpose of Hellhounds. As creator of other types of shadowspawn, he used his knowledge of the hellhounds to transform the souls of wolves into an actual animal he could control. Thus emerged the notorious Darkhound: the assassins of the Dark One.