The term 'cereal killer' is a play on words, referring to someone who is particularly passionate about cereal. The crossword solution for this term is 'ERGOT, ' which is a type of fungus that grows on grains such as rye and wheat. ERGOT has been used in medicine for centuries, but it can also be toxic if consumed in large amounts. In the past, ERGOT poisoning was linked to mass hysteria and hallucinations, leading to the term 'cereal killer.'