The Mule is the main villain of the second and third Foundation books by Isaac Asimov. He is a unique mutant with the incredible power to brainwash people through manipulation of their emotions, making them into his willing servants. This ability allows him to singlehandedly and effortlessly conquer civilizations without any bloodshed, and makes him a serious and genuine threat to the “Seldon Plan”.
The Mule is initially said to be an incredibly large and powerful warlord by his men, and his actual powers and identity are not known until much later on, while the Mule himself seemingly does not appear until near the very end of ‘’Foundation and Empire’’, where it is shockingly revealed that the Mule is, in fact, Magnifico, a comic relief character previously known to be the Mule’s runaway jester, and had been tagging along with the protagonists in this innocent guise in order to learn the location of the elusive “Second Foundation”, which the main characters themselves did not know and were trying to discover and reach it before the Mule (who already had subdued the First Foundation) could. However, Bayta figures all this out just in time to shoot the psychologist Ebling Mis dead right before he can reveal where the Second Foundation is.
In the third book, the Mule has conquered most of galactic civilization and established an empire, while still searching for the Second Foundation, but is ultimately thwarted when the protagonists find a way to nullify his powers.