Out of the unimaginable blackness beyond the gangrenous glare of that cold flame, out of the tartarean leagues through which that oily river rolled uncanny, unheard, and unsuspected, there flopped rhythmically a horde of tame, trained, hybrid winged things that no sound eye could ever wholly grasp, or sound brain ever wholly remember. They were not altogether crows, nor moles, nor buzzards, nor ants, nor vampire bats, nor decomposed human beings; but something I cannot and must not recall. They flopped limply along, half with their webbed feet and half with their membranous wings; and as they reached the throng of celebrants the cowled figures seized and mounted them, and rode off one by one along the reaches of that unlighted river, into pits and galleries of panic where poison springs feed frightful and undiscoverable cataracts.
—H. P. Lovecraft, "The Festival"
The Byakhee often serve Hastur the Unspeakable. Since the beings are made of ordinary matter, they can be injured by normal weapons such as firearms. Byakhee can fly through space and can carry a rider, though the rider needs protection from the vacuum and cold by suitable spells or potions. One hypothetical form of protection from interstellar space is an elixir known as space mead, whose side effects are unknown. Byakhee live in interstellar space but may be summoned to Earth to perform tasks or to serve as steeds.
Call of Cthulhu [the role-playing game] has taken its description of the byakhee from the creatures in Lovecraft's "The Festival", but it is uncertain whether Lovecraft and Derleth's creations are one and the same.
—Harms, Encyclopedia Cthulhiana, pp. 42.