Shadeling

Left to itself a shadeling is a drifting, thinking patch of darkness about as big as a human, though its outline never quite settles into any shape. People often liken them to doppelgangers, and with reason: both copy others and then murder them.

A shadeling attacks by flowing over its target's shadow. If the victim fails a save vs. breath weapon, the shadow is engulfed and the shadeling becomes a solid copy, complete with hit points, Armor Class, physical attacks and defenses, weapons and armor. Copied magical gear looks right but carries no magic. The duplicate then tries to kill the original. If it succeeds it keeps the stolen shape for 2d10 days before slipping back to shadow and searching for someone new. Slaying it while it wears a borrowed body only forces it back into shadow form and robs it of duplication for a full day.

In shadow form it is immune to physical harm and shrugs off most magic, but there are answers. Light or darkness spells have a 5% chance per caster level to destroy one, and their continual versions a 10% chance per level. Dispel evil and protection from evil work normally. Shadow walk or sunray cast on a shadeling destroys it outright. Holy word drives a solid shadeling back into shadow, and against one already in shadow form it banishes the creature to the demiplane. Though they look undead, shadelings cannot be turned; they are elemental beings, vulnerable to magic that affects Inner Planes creatures.

Shadelings exist, it seems, only to copy the living and feed on their shadows. Groups of up to 10 appear, yet they show no teamwork, rank or plan, and may all pile onto the same victim at once. Sages suspect they cross over together through elemental vortices, which would explain why they travel in numbers at all.

They have no niche on the Prime Material Plane, and their role on their home demiplane is unknown. Wizards prize them for one trick: a mirror enchanted with enchant an item and permanency, then engulfed by a shadeling, instantly becomes a mirror of opposition.