Invisible Stalker

This spell summons an invisible stalker from the Elemental Plane of Air. [...]

This 8 hit die monster will obey and serve the spell caster in performance of whatever tasks are set before it. The invisible stalker will follow instructions even at hundreds or thousands of miles distance. 

However, while the creature is bound to serve; it does not do so from loyalty or desire. Therefore, it will resent prolonged missions or complex tasks, and it will attempt to pervert instructions accordingly.  

The material components of this spell are burning incense and a piece of horn carved into a crescent shape.