Power keys are very, very rare items created by deities and are the cleric's equivalent of wizard's spell+keys. They cause divine spells to work as normal despite planar spell alterations, even maximizing their effects. A power key can change whenever the deity wills, so there's no guarantee it'll last forever.
A power key might be anything - it might be a certain rune or an action like plucking the feathers from a chicken. They may work for only one spell, or for a whole sphere of spells.
Many evil powers enjoy making false keys which may do nothing or cause a spell to work according to some malevolent twist.
Source
Planescape Campaign Setting
The Planewalker's Handbook
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