Fey Promises

Your word is your bond. Their word is their existance.

— Astrid the witch

In the grand landscape of all things, the fey folk value truth above all. Coming from a world of pure light and refraction, the ability of those of the other realms to lie so well does vex the average Fey. With the fact that most Fey you will meet are beings of light stuffed into bodies of flesh, the longer a fey has been in the tangible world the better they will become at both lying and bending the truth.
But at its core is the Fey Promise - a monkey's paw, wrapped in a 'careful what you wish for', a promising something to a fey or getting them to promise something to you is a lifelong binding contract until that comes to pass, and the fey will make it come to pass. There is something inhuman in the way they will become so dogged and determined to make a promise come to pass - even at their own detriment. There are myths of Fey folk hunting down a person just to get them to kill said fey just because there was a promise to kill the fey.
Conversely, promising to a fey will cause them to do the thing promised to the best of their ability, even if that takes millenium, even if the spirit of the thing is left waiting behind the exact wording of the thing.
Many a story across the realms sees lawyers, and dealmakers and hunters twist themselves into knots to get the right wording on a thing, missing the core concept behind these promises - honesty and sincerity. Truely and frankly believing in what you are trying to get a fey to do, or do with a fey is the best way of getting what you desire, and the need to twist a promise to your advantage has been one of the easiest ways of ending up with a wish you do not desire.