A phase spider was a magical beast akin to a predacious version of the ethereal filcher. Phase spiders were monstrous arachnid-kin that dwelt within the Ethereal Plane. They made quick forays into the natural world to snatch an unlucky creature to feed upon. Those stolen by phase spiders were never seen again and were regarded as unsolved disappearances.
The phase spider resembled a large, two-eyed, spindly legged spider with the some dark blue and white coloration.
While the filcher transported between the Ethereal Plane and the Material Plane for the purpose of stealing, the phase spider did this for the purpose of hunting. It looked for a likely spot in the Ethereal Plane, and then teleported (its form of teleportation was known as “phasing,” hence the name) to the Material Plane, presumably ambushing travelers it expected to be there, biting and killing them, and then quickly retreating back to the Ethereal Plane.