There is something I always avoided using like a plague, and it is power keys. I simply can't figure it out.
In the description of planes in the different "planes of.." (me no use produces about the planes after Planescape) they always describe the power keys to a plane as givent by the power from that plane, and only to their own priests. That does not make any sense! A priest doesn't need a power key in the homplane of her own power! At the same time, the planescape setting and other product describe power keys as working only on the specific plane they are designed for.
What's the right and what's the wrong in this all?
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about slaying a fiend out of his home plane: I recall they just reform back there, but can't put my hand on wher it's written down. Is it all the fiends? or only some of them. How long does it take? are they stuck in their home plane for some times afterward?
On the fiends; all the stuff you need is in Faces of Evil. It depends on the fiend, and specifics are detailed in each race's entry.