The beholder is a deferred feature in NetHack 3.6.0, a powerful monster in the eye/sphere class. Code for the creature appears in the source code but as of version 3.6.0 is commented out, so the monster will not be encountered in the Mazes of Menace.
The Beholder is the quest nemesis for the Neutral Quest. It holds the Key of Neutrality, one of the three Alignment keys; you need two of these keys to unlock the indestructible doors in Vlad's Tower.
If the Beholder is killed by means that would not prevent it from leaving a corpse—in other words, not by stoning, sliming, or disintegration—it leaves behind the Eye of the Beholder instead of a regular corpse. Since the Beholder is not difficult to kill by physical or magical means, it should be easy to kill it in such a way as to get the Eye.
The Beholder combines the gaze attacks of various creatures: the catoblepas (instant death), bloodshot eye (stun), umber hulk (confusion), and chasme (sleep). It also has a damaging gaze attack which deals up to 24 damage.
The easiest way to defeat the Beholder is to put on a blindfold or towel before entering its room, which will block all of its attacks. Even if you don't have telepathy, the Beholder is easy to find if you close the door behind you. Reflection will only protect against the gaze of death: blinding yourself is almost always preferable.
Important Note: There is an interesting quirk (possibly bug) about the Beholder's death gaze attack when you have reflection - it will not be reflected at all if the Beholder cannot see you (for example, if you are invisible). With other gaze attacks what usually happens is that you are protected from the effects of the gaze but the monster is not hit by it (because it can't see the reflection), but in the case of the Beholder's death gaze, it simply bypasses reflection entirely if it cannot see you. This means that if you are invisible, reflection will not protect you from instant death.
You'll know it has been defeated when the game says "All that remains is a single eye..."
The beholder (A.K.A the eye tyrant, the sphere of many eyes)
is most frequently found deep underground, although infrequently
it will lair in desolate wilderness. The globular body of this
monster is supported by levitation, and it floats slowly about
as it wills. Atop the sphere are ten eyestalks, while in its
central area are a great eleventh eye and a large mouth filled
with pointed teeth. Each of the eyes is said to have a different
magical property, all of which are deadly or malicious. The
beholder is hateful, tricky, and avaricious and is known to have
a fondness for both traps and artifacts.
Beholders in dNetHack attack twice per round with random gaze attacks, and share the floating eye's paralysis gaze counterattack.
Since the random gaze attacks include death and petrification gazes as well as damaging elemental gazes, it is best not to go near one without reflection and a blindfold.
"Beauty in things exists merely in the mind which contemplates them."
In GruntHack, beholders pose a significant threat. They usually generate asleep, but when awake, they can instapetrify you, disintegrate you (the same kind of disintegration as gods do) and cancel your inventory. They're prime targets for genocide in Grunt. As in other variants, covering your eyes makes it nearly harmless with the exception of a relatively weak bite attack. Note that none of its attacks respect reflection.
The beholder is a well-known creature from Dungeons & Dragons, one of the franchise's most iconic original creations. As described in the SLASH'EM encyclopedia entry (taken from a D&D manual), beholders have spherical bodies with a single large eye, ten smaller eyes on stalks above it, and a mouth full of teeth. The smaller eyes are used for offense or defense; each one has a different magical ability. Beholders are typically hostile to other creatures, and often hostile to one another.
The artifact Eye of the Beholder is a pun on the saying "Beauty is in the eye of the beholder". (See the dNetHack encyclopedia entry, which quotes a similar phrase, though there is no artifact eye in that variant.) The word "beholder" is a less common (often poetic) synonym for "observer".