High Stakes - A Whedonverse Campaign
Powerful magical beings capable of warping reality seemingly at a whim.
They seem to have been banished from the material plane some time ago, although the details of how or why are unclear.
However, they are only able to interact with our plane via the will of one who would summon them, and can only influence the world subject to the specific wording of the wish.
How they interpret the wording of the wish, however, is what makes Djinn dangerous.