From website - https://en.wikipedia.org/wiki/Amanita_phalloides - Amanita phalloides (/æməˈnaɪtəfəˈlɔɪdiːz/), commonly known as the death cap, is a deadly poisonous basidiomycetefungus, one of many in the genus Amanita. Widely distributed across Europe, but introduced to other parts of the world since the late twentieth century ....A. phalloides is one of the most poisonous of all known mushrooms. It is estimated that as little as half a mushroom contains enough toxin to kill an adult human. It has been involved in the majority of human deaths from mushroom poisoning, possibly including Roman Emperor Claudius in AD 54 and Holy Roman Emperor Charles VI in 1740. It has been the subject of much research and many of its biologically active agents have been isolated. The principal toxic constituent is α-Amanitin, which causes liver and kidney failure.
From website - 7 of the World's Most Poisonous Mushrooms - Notable deaths include Pope Clement VII, who died of accidental death cap poisoning in 1534, and possibly Roman Emperor Claudius in 54 CE.