A bit on negative effects of sensory deprivation on Wikipedia:
Studies have been conducted to test the effect of sensory deprivation on the brain. One study took 19 volunteers, all of whom tested in the lower and upper 20th percentiles on a questionnaire which measures the tendency of healthy people to see things not really there, and placed them into a pitch black, soundproof booth for 15 minutes. After, they completed another test, which measures psychosis-like experiences which was originally used to study recreational drug users. Five people reported seeing hallucinations of faces, six reported seeing shapes/faces not actually there, four noted a heightened sense of smell and two people reported sensing a "presence of evil" in the room. Not surprisingly, people who scored lower on the first test experienced fewer perceptual distortions, however, still reported seeing a variety of delusions and hallucinations.
According to the Journal of Nervous and Mental Disease the hallucinations are caused by the brain misidentifying the source of what it is currently being experienced, a phenomenon called faulty source monitoring."
Telling him to do the same will most likely cause him at the least to sense something with him that just isn't there. Not a productive plan.
It's really got nothing to do with depriving someone of their visual senses so that they can imagine things, its just a condition to the summoning.
I mean, would a cold, malicious, evil demon appear on a sunny day on a beach in a swimsuit and try to scare you in front of a thousand people or would it rather haunt you in a secluded, dark place where your alone?