Key 17 is a potent hallucinogen with highly specialised effects.
It causes the user to experience written signals as the objects themselves – a coffee mug reading ‘world’s greatest dad’ would be perceived as the user’s father, for example.
Key 17 is used as a means of interrogation by the agents of the Outer Church, who use it to substitute for actual torture by creating the illusion that such torture has taken place. Key 17 also appears to have a secondary effect (common to hallucinogens in general) of messing with the user’s sense of time’s passage.
A similar drug called Key 23 also exists – it appears to be a later refinement of of the original.