The byakhee is a type of monster introduced in SLASH'EM, and it also appears in dNetHack and SlashTHEM. Byakhees are much stronger members of the B bat or bird monster class, and move erratically like the other monsters in that class.
A byakhee has sleep resistance, poison resistance and cold resistance; they possess regeneration, flight, are breathless, and can follow you to other levels. Byakhees have four melee attacks, including a sting that can stun you and is unaffected by magic cancellation.
Byakhees are always generated hostile, and may appear in small groups.
From the mid-game on, byakhees represent a troubling threat due to their 20 speed and stun-inducing melee sting, making it possible for them to wear you down while you either whiff attacks from being stunned or spend turns trying to cure it. Additionally, their regeneration makes killing them difficult if you cannot deal enough damage consistently, even when using Elbereth. While not on par with the greater golems, they can remain nuisances well into the late game.
Strong and consistent forms of offense, such as well-enchanted weapons, are best for dealing with byakhees; funneling them down halls may be ideal due to their scattered movement and tendency to appear in groups. Their MR score of 25 also leaves them fairly vulnerable to magic and other effects - in particular, charming or taming them may work in your favor, as byakhees are decent options for pets due to the speed, regeneration and stun attacks that make them such a danger.
The byakhees (or byakhee birds) originate from the H.P. Lovecraft short story "The Festival", and were later featured in August Derleth's tale "The House on Curwen Street". They are servants of the Great Old One Hastur, and their description in "The Festival" paints them as four-limbed eldritch monsters with bat-like wings, possessing visual traits of various animals such as vampire bats, crows and buzzards, as well as somewhat resembling human corpses. The text specifically describes them as "not altogether crows, nor" any of the other descriptors used, and their otherworldly, eldritch nature is emphasized by the speaker's difficulty in fully recalling their appearance.
A creature of the stars, this bat like being can be summoned
across the vast interstellar gulfs to serve magicians and
dark powers. It is said that they will seize those who
summon them and cannot best them and carry them off to whatever
terrible fate awaits them on other worlds.