This tiny creature appears to be a humanoid skull with eight spidery legs.
Skull Spider CR 1/4
Speed 20 ft., climb 10 ft.
Melee sting +0 (1d2-2 plus poison)
Space 2-1/2 ft.; Reach 0 ft
Organization swarm (2-20)
Skull spiders are tarantula-like creatures that reside in the skulls of their victims. The two front legs of a skull spider contain poisoned barbs that they use to sting their victims. The weak and fleshy body of a skull spider is about the size of a grapefruit and is easily damaged. Its eyes grow on the end of long, slender stalks. Skull spiders take up residence within skulls as a means of protecting themselves in a manner similar to hermit crabs.
Their eyestalks protrude through the empty eye sockets of their skull, and their legs have a backwards curve in the first joint that enables them to extend out of the bottom of the skull to allow rapid locomotion. Skull spiders can also fold their legs under their skull so they cannot be seen. Many an adventurer has been unnerved by the sight of dozens of skulls seemingly sprouting long, spidery legs and skittering toward them.
A colony of skull spiders is led by a king and queen, which are the only two members of the colony that are capable of reproducing. After a victim is subdued, the queen deposits an egg in the skull. Queen skull spiders are always 3 HD. The larva hatches, consumes the brain over a period of weeks, and then enters a pupae stage. After several months, when the corpse is sufficiently deteriorated, the new skull spider hatches, uses its strong legs to detach the skull, and goes to join its colony.
Skull spiders always attack en masse, swarming over their victims in great numbers and stinging them repeatedly.
Skull Spider from the Tome of Horrors Complete, Copyright 2011, Necromancer Games, Inc., published and distributed by Frog God Games; Author Erica Balsley.