The Beholder combines the gaze attacks of various creatures. Namely, the catoblepas (instant death), bloodshot eye (stun), umber hulk (confusion), and chasme (sleep). It also has a 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.
You'll know it has been defeated when the game says "All that remains is a single eye..."
Encyclopedia entry Edit
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.