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The caster makes it impossible for a single creature to speak about a single topic.
The topic should be as narrow as possible for the spell to work properly.
An appropriate topic might be events that occurred while the creature was in captivity, the whereabouts and names of tormentors, the secret location of a hidden treasure, the password into a guarded compound, the command word of a magic item, or the explanation behind some major occurrence.
Invalid topics include verbal components of spells, a creature's whole life history, or anything too broad for a reasonable definition of a single topic.
Whether a topic is too broad is up the DM, who can double the corruption cost and have the spell automatically fail if the caster persists in attempting an invalid topic.
Thus, this can be a dangerous spell to attempt.
The affected creature cannot communicate regarding the topic in any way.
Speech becomes gibberish, writing is reduced to an indecipherable scrawl, gestures are impossible, and even telepathy (including detect thoughts) is scrambled.
The creature has not forgotten the information, however, and it can be a maddening occurrence for the creature to be unable to communicate information that might be of life-and-death importance to friends and allies.
Corruption Cost: 1d6 points of Strength damage.