INVISIBLE STALKER

The invisible stalker is a creature from the elemental plane of air, normally encountered on the Material plane only due to the conjuration of some magic-user. This conjuration causes the creature to serve for a period on this plane.

Invisible stalkers roam the Astral and Ethereal planes, and if they are encountered there [or] on the Elemental planes, they can be dimly seen. Otherwise or unless their opponents are able to detect/see invisible objects, their invisibility causes opponents to subtract 2 from "to hit" dice rolls.

The conjuring party retains full command of the invisible stalker summoned until it fulfills its duties or is killed. Once set upon a mission, an invisible stalker will follow through unceasingly until it is accomplished. They must be destroyed to make them cease an ordered attack.

They are faultless trackers within one day of a quarry's passing.

Once a mission is finished, the creature is freed to return to its own plane.

The invisible stalker is at best an unwilling servant but will not resent a brief, uncomplicated task. Service involving a period of a week tries the creature severely, and anything longer is certain to make it attempt to fulfill the letter of command by perverting the spirit of it. This is not to say that impelling the invisible stalker to serve for extended periods is impossible, but the compulsion to cause it to do so fully and properly must be great, i.e. a carefully worded command from a very powerful magic-user. 

A simple command such as "Follow me, and guard me from any attack," could be interpreted to mean follow at 100-ft distance if the invisible stalker had been on duty over a week — or perhaps even if it hadn't been that long, for dealing with such creatures is always a hazard.

Similarly, a command to: "Keep me safe from all harm," can be construed by the invisible stalker to mean that it is to carry the conjuring party to its own plane and place them in suspended animation in a secret room in its own abode, thus carrying out its duties to the letter.

Each day of duty which an invisible stalker serves will see a 1% cumulative chance of the creature seeking to pervert the intent of its commands in order to be free of servitude. If no option remains open, the stalker must continue to serve.