Psurlons are malevolent, wormlike creatures that live on the Astral Plane. A fully grown specimen is 7 feet long. They have legs that end in hooves and arms that end in hands with three long fingers. A psurlon's eyeless head resembles that of an earthworm, capped by a maw ringed with teeth. Psurlons adorn themselves in richly colored robes and rarely wear armor or carry weapons.

Psurlons live for thousands of years because they spend most of their time in the Deep Astral. Every hundred years or so, psurlons leave their astral strongholds, invade Wildspace systems, and indulge in a seven-year-long ceremony called the Feast of Worlds, during which they consume as many sentient life-forms as they can before returning to the Astral Plane. Psurlons prefer the flesh of humans and halflings but don't mind feasting on other folk. They use their spellcasting abilities to infiltrate the settlements of their intended victims.

On the Material Plane, psurlons have been known to work with mind flayers. Together, they collect victims to feed on; the illithids devour the victims' brains while the psurlons consume the rest of the prey. Githyanki despise psurlons because of this alliance and attack psurlon strongholds in the Deep Astral wherever they are found.

When a psurlon dies, other psurlons store the corpse in a safe place. As the corpse decays, the psurlons lay one or more eggs inside it. These eggs hatch 24 hours later, each one producing a Tiny worm. For the next seven days, the worms feed on the corpse and on each other until only one remains. This worm crawls out of the putrescent remains of its dead host as an adult psurlon.

One out of every hundred psurlons is a mutant with two heads, one at each end of its body, and a superior intellect. Other psurlons look to the two-headed ones for leadership.

A psurlon can use magic to assume the form of a specific Medium Humanoid. First, the psurlon must consume the creature it wants to imitate. It then enters a psionic trance for 8 hours, at the end of which it takes on the appearance of the creature it ate. The psurlon gains that creature's memories and languages, but none of its class features or other abilities. The transformation is permanent and can be undone only by a wish spell. Despite appearances, the psurlon ringer is still an Aberration, and other psurlons recognize it for what it is.