Cloakers are a species of aberration from Dungeons & Dragons, and one of the many, many examples of Gotcha Monsters to come out of the days of Advanced Dungeons & Dragons. Described charitably as something like a land-dwelling, flying manta ray with psionics and human-plus level smarts, Cloakers mostly attract derision because their entire traditional schtick is pretending to be magical cloaks and then killing anyone stupid enough to try and put them on.
The cloaker was introduced in the first edition Advanced Dungeons & Dragons game adventure module, Secret of the Slavers' Stockade, as an ally to the adventure's antagonists. The cloaker subsequently appeared in the first edition Monster Manual II, and then appeared several times in the game's second edition, third edition, fourth edition and fifth edition.
In the adventure Secret of the Slavers' Stockade, a cloaker (or "Tenebra Complexor") first appears as the guardian of a holding room in which the Slave Lords keep their newly acquired slaves to make them docile. The cloaker clings to a wall, hiding among actual cloaks and blankets, resembling a semi-circular cape with a long mace-like tail, and has two claws at the ends of the "cape". Numerous round black, button-like eye spots help complete the illusion; when the cloaker conceals its tail and claws it is hard to distinguish it from a real cloak. A cloaker has no head, but in the middle of its belly is a mouth and two red, glowing eyespots; its rear eyespots make it so that a cloaker has no effective rear to attack. This cloaker will allow characters to enter the room and approach within the range of its moaning ability.
The module notes that the cloaker is a "rare and exotic creature that typically lives far underground", and that "How the slavers managed to obtain the services of this bizarre creature is unknown, but the beast is believed to be intelligent." As part of this encounter, the module also describes the abilities of a cloaker. A cloaker regularly emits a subsonic moaning, which can cause any of several different effects as the monster pleases by varying the moan's intensity. The lowest level, which it uses on the slaves appearing in the encounter, causes nervousness and unease and numbs the minds of those who listen for too long. The slaves have been exposed long enough that they will not react to other characters in any way. The second level of intensity causes fear in characters, the third level causes nausea and retching, and the final intensity can cause a single character to be held motionless.
The scenario describes how a cloaker will fly at a character which it has subdued with its moaning, and envelop its victim in its folds, pinning the victim's arms to his side and biting the victim. Any characters trying to help the victim will be attacked by the cloaker's tail, and any character harming a cloaker wrapped around a victim is likely to harm the victim as well. A cloaker also has the power to manipulate shadows, and can use them to cover a character's face or surround itself with shadow to hide; a cloaker can also create images out of shadows, including doubles of itself to misdirect opponents.
A full description of the monster, including game statistics, appears at the back of the module to complete the creature's depiction. This section notes that a cloaker is chaotic neutral in alignment, and describes a cloaker as "a shadow-dwelling, unearthly creature. They are normally only encountered in deep, dark caverns, far beneath the earth. Though they are highly intelligent, their thought processes are alien to most races and usually only magic-users are able to communicate with them."
A variant called a sea cloaker appeared first in the Lankhmar: City Of Adventure set (1985).
The cloaker first appeared for second edition Advanced Dungeons & Dragons in the initial Forgotten Realms appendix of the Monstrous Compendium series (1989). In this set, the creature receives a detailed description: "When a cloaker is first seen, it is almost impossible to distinguish this monster from a common black cloak. The monster's back has two rows of eye spots running down it that look much like buttons, and the two ivory-colored claws on its upper edge can easily be mistaken for bone clasps.
When it unfurls itself and moves to attack, however, its true nature becomes all too obvious. At this point, its white underside is clear and the monster's face is fully visible. This face, with the glow of its two piercing, red eyes and the needle-like fangs that line its mouth, is a truly horrible sight. At this point, the monster also uncurls the whip-like tail at its trailing edge and begins to swish it back and forth in anticipation." The cloaker's entry is reprinted in the Monstrous Manual (1993), which states that the cloaker is related to another dungeon-dwelling creature known as the trapper.
The sea cloaker is detailed again in the adventure module LNA2 Newhon (1990), Lankhmar: City of Adventure (1993), and Fritz Leiber's Lankhmar (1996).
The cloaker lord first appears in the Forgotten Realms setting boxed set, Menzoberranzan (1992), in the booklet "Book One: The City". The cloaker lord is described as a superior subrace of the cloaker, larger and more intelligent than a cloaker, with a bat-like form. A cloaker lord has the ability to naturally dominate a lesser cloaker, and in the events depicted in the boxed set, the cloaker lords of Menzoberranzan "have recently come to rule their lesser brethren, drawing normally-solitary cloakers together into loose raiding bands, and forcing other monsters ... into servitude." This entry also mentions that a cloaker lord near death can devour a cloaker, and if the cloaker lord survives a few more days it will split apart, "giving birth" to a new cloaker lord and a number of baby cloakers.
Three cloaker variants for the Ravenloft setting are introduced in Ravenloft Monstrous Compendium Appendix III: Creatures of Darkness (1994). The shadow cloaker is described as a parasite believed to have originated from the Demiplane of Shadow that attaches to another creature, and uses this host to drain the life force of other creatures. The resplendent cloaker is described as a "benign symbiont" that gives off a dazzling glow, and somehow feeds by healing the wounds of its host; however, if injured, the resplendent cloaker will drain hit points from its host to heal itself. The undead cloaker is described as a Chaotic Evil creature believed to be the undead remains of a resplendent cloaker that has had its life drained away by an undead creature; what appears to be a tattered cloak is really the creature’s rotting flesh. An undead cloaker attaches itself to a host, and uses that host to drain the life energy from living creatures, turning them into zombies.
The cloaker appears in the Monster Manual (2000) for the game's third edition. In this edition, the cloaker is given the aberration creature type. This book describes cloakers as hunting together in small flocks, and having the ability to emit a subsonic moan that can unnerve, frighten, nauseate, or put foes into a stupor. They'd later be reprinted in the 3.5 Monster Manual 1.
The cloaker lord appears in this edition in Monstrous Compendium: Monsters of Faerûn (2001). In this edition, a cloaker lord is able to shift to the Plane of Shadow at will, and can cast spells as if it were a powerful wizard. This entry also describes Rringlor Noroth, a cloaker city in the Underdark of the Forgotten Realms; such cities are established when cloaker lords draw the cloakers together. Rringlor Noroth is ruled by twelve cloaker lords that can join their bodies together to form an orb of living shadowstuff called a "Conclave of Shadows"; this unity of thought helps them quickly reach consensus in decisions of governance.
Lords of Madness: The Book of Aberrations (2005) introduces the shadowcloak elder, a ray-like creature whose body ripples in shades of black, gray, and pallid white. The book states that these creatures are the perceived leaders of the underground cloaker cities, "cloakers of great size and power, tainted with the substance of shadow and skilled in dark sorcery." A shadowcloak elder can cast spells as a sorcerer, disappear into shadows, and shift to the Plane of Shadow.
Cloakers actually didn't show up until the third Monster Manual in this edition.
Cloakers returned to the Monster Manual with the release of D&D 5e.