Ergot
Ergot is a fungus which grows parasitically on drier grain plants such as rye, wheat, and barley (though not rice). Sometimes the black jutting heads of ergot reveal its presence, but other times it can have the appearance of discolored grains.
Ergot is a significant threat to grain farmers, as it reduces yields and has a dangerous effect if consumed. In years of plenty, ergot-infected grain can be discarded, but in times of poor harvest or poverty those desperate for food can ill-afford to throw away grain which has come into contact with ergot.
When consumed, ergot-infected grain (or foods made from the same) inflicts spasms, nausea, vivid and terrible hallucinations, and manic or psychotic behaviour. This is followed by loss of feeling in the extremities and then the onset of gangrene. Between this and self-destructive panic induced by the hallucinations, ergot is most often fatal, and when it afflicts one person often it will afflict several eating from the same food source.
In the Realm, the Benevolent Stewards of Health and Utility take extensive measures to ensure ergot does not afflict the grain supply, and the penalty for selling ergot-infected grain is death.