The phase spider is a monster that appears in some variants.
It's yet another monster that was borrowed from Dungeons & Dragons.
The D&D incarnation resembles a large white spider with dark green patterns, and a humanoid head with two eyes. It's an ambush predator, phasing from a parallel dimension to surprise its prey, hence the name.
Phase potion, which is related to oil of etherealness, is more
useful for general combat. When imbibed, this potion allows the
user to shift in and out of phase with the Prime Material plane
at will, much like a phase spider. When out of phase, the user
is impervious to all forms of attack except those that reach
into Ethereal plane. ... Phase potions are brewed from phase-spider
ichor or from the concentrated juices of rare underground fungi.
[ RPG Sheets by Amalor Mymnyx ]
Phase Spiders are not true Spiders, but an alien race which
appear as giant spiders with humanoid shaped heads. Phase Spiders
speak a whispery language which is reminiscent of the sound of the
wind blowing through the trees. ... Phase Spiders are considered
Enlightened creatures as they have the ability to phase in and out
of our normal space-time into what most scientists would refer to
as Void-Space. Phase Spiders use this ability to their great
advantage in combat.
[ Alien Update Lexicon by Randy Walker ]
Phase spiders favor hit-and-run attacks, biting you and then teleporting away to prevent retaliation. This tactic is of questionable use against poison resistant characters, however. Their attack can teleport you to a random location (subject to teleport control)
In addition to being able to teleport, phase spiders in dNetHack can phase through walls.
Sometimes confused with neogi or driders and called planar
spiders on some worlds of the Prime, phase spiders are intelligent
web-spinners with a raised thorax and a humanlike
head. What really makes phase spiders stand out is their
enviable ability to phase back and forth between a plane and
the plane's Border.
Unknown to most creatures, phase spiders possess
large web habitats on the Ethereal (both in the Deep and on
the Border) and only appear on other planes while hunting
for food. Some phase spiders do live mostly on material
planes, but these spiders are the exception, not the rule. Like
many more familiar cultures, Ethereally-based phase spiders
are not a cohesive race, but instead exist in separate clutches
(tribes), each of which claims various Prime territories as its
sole hunting ground. Sometimes disparate phase spider
clutches cooperate, but sometimes they come into conflict
over temtorial disputes.