Ephemeral hangman

An ephemeral hangman was an abnormal aberration of the Plane of Shadow that could pass through shadows and spaces much too small for its great size in search of unsuspecting and vulnerable prey.

Description
Its form was little more than a mass of dozens of writhing black tendrils or tentacles, all entwined in a trunk-like body and with a gnashing fang-filled mouth at their heart. Appearing black as ink and nigh-unseen in darkness, it seemed to be made of pure shadow, yet it weighed almost. Usually standing erect to a height of, it appeared like an alien and animated tree. However, it could extend its tendrils out and lay itself flat like a shadow upon the floor. A victim's first sight of one could be seeing a patch of shadow open and uncoil, almost like a flower, into tentacles and mouth.

Personality
An ambush predator, the ephemeral lurked in small and dark places and waited to seize unsuspecting prey. It preferred living creatures and especially those too small and weak to defend themselves, even children. They didn't interact much with those they weren't devouring, but their personalities varied: one might be a picky eater and focus on the biggest target in a group, another would be greedy and try to take as many as it could.

It despised bright light, as it limited its shadow-borne powers. If exposed to sunlight or a daylight spell, it would try to escape by whatever means it still had.

On the rare occasions they did speak, ephemeral hangmen were known to use a certain dialect of Undercommon.

Abilities
By entering a kind of "shadow pocket", an ephemeral hangman could pass through spaces too small to normally contain it. It could fit its bulk inside a space only wide without impediment, and even as small as  wide, but was hampered. This ability helped it hide, but it could only be done in darkness or shadow. If a bright light (more than a torch or light spell) shone on it while in its shadow pocket, the ability failed and the ephemeral hangman was forced into the closest able to contain it, dealing it damage according to the distance moved.

An ephemeral hangman could transport itself instantly to a site up to away by stepping into a shadow and travelling via the Plane of Shadow, similar to dimension door or the step into shadow mystery of a shadowcaster. They could do this thrice a day, but of course only in a place of darkness or shadow.

An ephemeral hangman quickly healed any injuries it took, and it could even reattach severed tendrils in a moment or regrow them from the stump in up to 18 minutes. Only spells of light and searing light harmed it permanently. However, this regeneration halted in bright light, and then the ephemeral hangman could be harmed normally.

A strike from an ephemeral hangman's tendrils caused the target to be slowed, as with a slow spell, if they could not resist with an act of will. The effect lasted anywhere up to 72 seconds, though to a victim it would feel much, much longer while the hangman was constricting and eating them.

A creature of shadow, the ephemeral hangman was naturally very good at hiding and moving stealthily.

Combat
When it struck with its tendrils, an ephemeral hangman could immediately attempt to grab a creature much smaller than itself (that is, around human size or smaller). If the target could not free themself, the ephemeral hangman would quickly drag them into mouth and bite them, or else try to constrict and crush them—likely how it got its name.

Society
An aberration formed from pure shadow, ephemeral hangmen were abnormal even among the denizens of the Plane of Shadow. Nevertheless, they were native to that plane.

History
The origin of the ephemeral hangmen was unknown to all. A few sages theorized that they began as the Shadow Plane counterparts of a tentacled creature like the roper and that these "shadow selves" somehow achieved their own independent existence and sentience.

Several ephemeral hangmen appeared amidst the shadow tentacles in the Black Rift in the Plane of Shadow when the dark priestesses there performed a ritual to worship Shar. In the, they served the black dragon Despayr as guards, attacking all trespassers. They occupied a chamber wherein religious tapestries were displayed. Sometimes, they hunted in the surrounding Shadow Swamp. They were likely defeated when adventurers in service to Mystra came and ruined cult operations there in early Eleint of that year. Most shockingly, if the fight went against them, priestesses who sacrificed themselves upon the altar in the spiral pit would earn the favor of Shar, who would send an ephemeral hangman to avenge them—bursting bloodily from their cracked-open torsos.

Background
The ephemeral hangman's mass of tentacles and its apparently dimensional powers suggest a monster of Lovecraftian horror, while its gruesome emergence from characters seems inspired by 'The Thing''.