Invisible Stalker

(from 5th Edition Monster Manual - 2014):

An invisible stalker is an air elemental that has been summoned from its native plane and transformed by powerful magic. Its sole purpose is to hunt down creatures and retrieve objects for its summoner. When it is defeated or the magic that binds it expires, an invisible stalker vanishes in a gust of wind.

Directed Hunter. When an invisible stalker is created, it stays at its summoner’s side until it is given a task to perform. If an assignment doesn’t involve hunting down and slaying a specific creature or recovering an object, the magic that created the invisible stalker ends and the elemental is released. Otherwise, it completes the task, then returns to its summoner for more commands, forced to serve until the magic that binds it expires. If its summoner dies in the interim, the invisible stalker vanishes after completing its task.

An invisible stalker is an unwilling servant at best. It resents any undertaking assigned to it. A mission that requires significant time might drive the invisible stalker to pervert the intent of a command unless it is worded carefully.

Unseen Threat. Invisible stalkers are composed of air and are naturally invisible. A creature might hear and feel an invisible stalker in passing, but the elemental remains invisible even when it attacks. A spell that allows someone to see the invisible reveals only the invisible stalker’s vague outline.

Elemental Nature. An invisible stalker requires no air, food, drink, or sleep.

(from Monster Manual v3.5 - 2003):

Invisible stalkers are creatures native to the Elemental Plane of Air. They sometimes serve wizards and sorcerers, who summon them to perform specific tasks. 

A summoned invisible stalker undertakes whatever task the summoner commands, even if the task sends it hundreds or thousands of miles away. The creature follows a command until the task is completed and obeys only the summoner. However, it resents protracted missions or complex tasks and seeks to pervert its instructions accordingly. 

Invisible stalkers have an amorphous form. A see invisibility spell shows only a dim outline of a cloud, while a true seeing spell reveals a roiling cloud of vapor. 

These creatures speak only Auran but can understand Common.

Combat: An invisible stalker attacks by using the air itself as a weapon. It creates a sudden, intense blast of wind that pounds a single target on the same plane as the creature. 

An invisible stalker can be killed only when it is on the Elemental Plane of Air. When performing a task elsewhere, it automatically returns to its home plane when it takes damage sufficient to destroy it. 

Natural Invisibility (Su): This ability is constant, allowing a stalker to remain invisible even when attacking. This ability is inherent and not subject to the invisibility purge spell. 

Improved Tracking (Ex): An invisible stalker is a consummate tracker and makes Spot checks instead of the usual Survival checks to trace a creature’s passage.

(from 2nd Edition Monstrous Manual - 1993):

The invisible stalker is a creature from the elemental plane of Air. Those encountered on the Prime Material plane have almost always been summoned by wizards to fulfill a specific task.

The true form of the invisible stalker is unknown. On the Material, Astral, or Ethereal planes, the invisible stalker can only be perceived as a shimmering air mass which looks much like the refraction effect caused by hot air passing in front of cold.

Invisible stalkers understand the common speech of men, but can not speak it. They can converse only in their own language, which sounds much like the roaring and whooshing of a great wind storm.

Combat: Invisible stalkers attack by using the air itself as a weapon. It is capable of creating a sudden, intense vortex that batters a victim for 4-16 (4d4) points of damage. Such attacks affect a single victim on the same plane as the invisible stalker.

Due to their invisibility, these creatures impose a -6 penalty on the surprise rolls of those they choose to attack. Similarly, all opponents who are unable to see or detect invisible foes are at a -2 on their attack rolls. Although they are fully invisible on the Prime Material plane, their outlines can be dimly perceived on the Astral or Ethereal planes.

Invisible stalkers can only be killed on the elemental plane of Air. If attacked on another plane, they automatically return to their home plane when their total hit points are exceeded by the damage they suffered.

Habitat/Society: Little is known about the lives of these creatures on their home plane. It is assumed that they are similar to normal air elementals when encountered there.

Those present on the material plane are there as the result of a conjuration by some wizard. This magic causes the creature to serve its summoner for a time. The conjurer retains full command of the stalker until it either fulfills its duties or is defeated and driven back to its home plane. Once given a task, an invisible stalker is relentless. They are faultless trackers who can detect any trail less than a day old. If ordered to attack, they will do so with great fury and will cease their efforts only upon their own destruction or the direct orders of their master. Once their mission is accomplished, the creature is free to return to its home plane.

The invisible stalker is, at best, an unwilling servant. It resents any task assigned to it, although brief, uncomplicated labors may be seen as something of a diversion and thus undertaken with little resentment. Tasks that require a week or more of its time will drive the invisible stalker to pervert the stated intent of the command. Such commands must be carefully worded and come from a powerful wizard. An invisible stalker may look for a loop hole in the command as a means of striking back at its master. For example, a simple command such as “keep me safe from all harm” may result in the stalker carrying the conjurer back to the elemental plane of air and leaving him there in a well hidden location. Each day of the invisible stalker’s indenturedness there is a 1% cumulative chance that the creature will seek a means to pervert its commands and free itself of servitude. If no option is open, the creature must continue to serve.

Ecology: Invisible stalkers are a species unwillingly transplanted to the Prime Material plane. They are slaves whose terms of servitude dominate their brief stays. Those who have been subjected to great hardship, assigned very difficult tasks, or who have faced death at the hands of humanoids, tend to retain a distrust or outright hatred of them. Those that have had an easy time during past periods of service or who are first time arrivals on the Prime Material plane may be easier to deal with. Such feelings may carry over to influence encounters with humanoids traveling in the aerial plane. Anyone who has befriended an invisible stalker in the past will find that voyages through the plane of elemental Air are far less hazardous than they might otherwise have been.

Invisible stalkers only obey those who actually summon them and few wizards can be commissioned to summon such a being on another’s behalf. Some mercenary wizards have been able to construct the necessary summons onto scrolls that are usable by others. These sell for between 5,000 and 10,000 gp and are very dangerous to use. Even the slightest error can cause users of such scrolls to come to a tragic end.