Medusoid mycelium is a deadly mushroom appearing throughout the "A Series of Unfortunate Events" collection, a series of dark but humorous, children's fiction. Daniel Handler wrote the series using the nom de plume Lemony Snicket.
Developed by the character Gregor Anwhistle, medusoid mycelium grows in the Gorgonian Grotto and wreaks havoc on main and secondary characters in several books, including "The Grim Grotto" and "The End." This particular mushroom is so poisonous that a single spore, eaten or inhaled, can kill its victim within an hour. Horseradish, in any form, is the only available antidote to treat the deadly effects of the mushroom. While mycelium is real, being the vegetative part of fungi, the medusoid mycelium is entirely fictional.