A phase spider is a monster added in SLASH'EM.
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 ]