Hallucination

Very much like real life, hallucination will cause you to see things that aren't there. Any objects will be seen as continually changing items of the same class, so a unicorn horn may be seen as a sack, key, but not as a sword. The most dangerous thing about hallucination is that it conceals what the actual monster is. For instance, "I see a fungus over there, I'll kill it real easy. Whack, whack. (Fungus replies) WHACK! DYWYPI? Foo, killed by a hallucinogen-distorted cockatrice." Easily cured by a unihorn.