# Beholder (monster)
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.
Beholders are implemented in the variants SLASH'EM (as a unique monster) and dNetHack and GruntHack (as a non-unique but rare monster), with some distinct properties in each.

Contents

1 SLASH'EM

1.1 Strategy
1.2 Encyclopedia entry

2 dNethack
3 GruntHack
4 Origin
5 References

SLASH'EM

e Beholder 

Difficulty

24

Attacks

Gaze 1d4 Death, Gaze 3d8, Gaze 1d3 stun, Gaze 1d3 confusion, Gaze 1d4 sleep

e Beholder 

Difficulty

24

Attacks

Gaze 1d4 Death, Gaze 3d8, Gaze 1d3 stun, Gaze 1d3 confusion, Gaze 1d4 sleep

Base level

15

Base experience

538

Speed

9

Base AC

−4

Base MR

70

Alignment

0 (neutral)

Frequency (by normal means)

Unique

Genocidable

No

Weight

500

Nutritional value

100

Size

Large

Resistances

None

Resistances conveyed

None

The  Beholder:

 can fly.

 has no limbs.
 has no head.

 regenerates HP quickly.

 does not eat. (*)
 is not a valid polymorphable form.

 is neither male nor female.
 is normally generated hostile.

 can follow you to other levels.
 is nasty.

 wants the Amulet of Yendor, the Bell of Opening, the Book of the Dead, the Candelabrum of Invocation, and your quest artifact.

 can be seen through infravision.

 never leaves a corpse.

Reference

SLASH'EM_0.0.7E7F2/monst.c#line4590

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.   
Despite its description, it does not have a bite attack and is, in fact, possibly the easiest alignment quest nemesis.

Strategy
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.[1]
You'll know it has been defeated when the game says "All that remains is a single eye..."

Encyclopedia entry

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.&nbsp; The globular body of this 
monster is supported by levitation, and it floats slowly about 
as it wills.&nbsp; Atop the sphere are ten eyestalks, while in its 
central area are a great eleventh eye and a large mouth filled 
with pointed teeth.&nbsp; Each of the eyes is said to have a different
magical property, all of which are deadly or malicious.&nbsp; The 
beholder is hateful, tricky, and avaricious and is known to have 
a fondness for both traps and artifacts.

[ Adapted by Ben Lehman from Monstrous Manual, by TSR, Inc. ] 

dNethack

e beholder 

Difficulty

10

Attacks

Gaze attack 4d6 random gaze type, Gaze attack 4d6 random gaze type, Passive 1d100 paralysis

e beholder 

Difficulty

10

Attacks

Gaze attack 4d6 random gaze type, Gaze attack 4d6 random gaze type, Passive 1d100 paralysis

Base level

6

Base experience

97

Speed

3

Base AC

4

Base MR

50

Alignment

−10 (chaotic)

Frequency (by normal means)

1 (Very rare)

Genocidable

Yes

Weight

1000

Nutritional value

1000

Size

medium

Resistances

fire, cold, shock

Resistances conveyed

A beholder:

 can fly.

 can survive underwater.
 does not breathe.

 has no hands.
 has no limbs.
 has no head.

 is poisonous to eat.

 does not eat. (*)
 is not a valid polymorphable form.

 is neither male nor female.
 is normally generated hostile.

 can be seen through infravision.

 appears only in Gehennom.

 cannot be tamed.

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.

A beholder corpse does not rot, offers a lot of nutrition, and grants clairvoyance, but is very heavy.

"Beauty in things exists merely in the mind which contemplates them."

[ Moral and Political, by David Hume ] 

GruntHack

e beholder 

Difficulty

11

Attacks

Gaze 0d0 slow, Gaze 2d25 sleep, Gaze 0d0 disintegrate, Gaze 0d0 petrification, Gaze 2d4 cancellation, Bite 2d4 physical

e beholder 

Difficulty

11

Attacks

Gaze 0d0 slow, Gaze 2d25 sleep, Gaze 0d0 disintegrate, Gaze 0d0 petrification, Gaze 2d4 cancellation, Bite 2d4 physical

Base level

6

Base experience

?

Speed

3

Base AC

4

Base MR

0

Alignment

−10 (chaotic)

Frequency (by normal means)

2 (Quite rare)

Genocidable

Yes

Weight

10

Nutritional value

10

Size

small

Resistances

cold

Resistances conveyed

A beholder:

 can fly.

 can survive underwater.
 does not breathe.

 has no hands.
 has no limbs.
 has no head.

 does not eat. (*)
 is not a valid polymorphable form.

 is neither male nor female.
 is normally generated hostile.

 can be seen through infravision.

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.

Origin
The beholder is a well-known creature from Dungeons &amp; Dragons, one of the franchise's most iconic original creations. As described in the SLASH'EM encyclopedia entry (taken from a D&amp;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".

References

↑ mhitu.c in SLASH'EM 0.0.7E7F2, line 2451