Phase spider

Phase spiders are a giant arachnid monstrosities that originate from the Ethereal Plane and can transport from Ethereal to Material Plane at will.

Appearance
These arachnid creatures were of a pale white and blue color, with spindly legs that reached out to a width of ten feet or more. They had giant dripping fangs and rows of beady red eyes.

Abilities

 * Senses: Darkvision.
 * Attack:
 * Bite: The phase spider can make a bite attack that deals piercing damage and additional poison damage (conditional on a saving throw).
 * Ethereal Jaunt: The phase spider can move between the ethereal plane and the material plane in either direction as a bonus action.

Known Phase Spiders
Mighty Nein encountered a phase spider in the sewers of Zadash.