Pain Worm

Pain Worms are monsters encountered in of Diablo II.

Lore
Pain Worms are spawned by Putrid Defilers, via parasitically infected their fellow demons. The worms immediately travel to the brain of the host monster, and grow fat on the vile thoughts of the fiend. If the monster is killed while the worms are gestating, the parasites eat their way through the beast's brain and ultimately burrow out of its skull in search of a new body to inhabit. It has been rumored that certain demon groups will simply imprison one of their brethren rather than risk killing the host and infecting another.

In-game
Their variants colors correspond to Putrid Defilers variants, but any Pain Worm can be spawned by any Putrid Defiler, it's based on location level on Normal.

Pain Worms (first variant) are unused because they spawn only if any Putrid Defiler variation is placed in a location of level 33 or lower.