Byakhees represent a threat primarily in the mid-game but can even trouble late-game character in SLASH'EM. They hit hard and fast enough to eventually wear you down, and their stunning makes them able to get in more hits as you try to cure it. Additionally, their regeneration makes them difficult to kill with multiple weak attacks from an Elbereth square, as they will retreat and heal before coming back. They are not on the level of the greater golems as a threat, but one should exterminate them with prejudice still.
Note that the same attributes that make them formidable foes (powerful attacks, high speed, regeneration, stunning) also make for great pets. Given that byakhees do not have particularly high magic resistance, they are easily charmed or tamed.
Byakhees come from the works of H.P. Lovecraft. They are supposed to live in space and be able to travel faster then the speed of light. They have four limbs and also bat like wings, that they use when they go down on a planet to feed.
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.