St Anthony’s Fire was one of the things that made the Middle Ages a horrible time in which to live. People would feel a pricking sensation in their arms or legs. This would turn to burning pain, and the arm would swell and redden before turning gangrenous and dropping off. You were lucky if the limb simply died without taking you with it.
Fun fact! Ergotamine, a chemical derivative of the ergot fungus on rye, is the major precursor for LSD.