I don't know where I first heard this story but it's one of the most important stories to me so I want to make an effort to retell it to the best of my abilities.

A solitary monk wandering through the desert came upon a city.
Seeking shelter for the night he went to the gate of the city and found there a demon.
The demon, smelling of feces and sulfur with peeling char black skin and wings of a bat, is asked by the monk what his business is standing outside the gate.
The demon replies that Satan had sent him to destroy the city.
"why only one demon" asked the wandering monk.
"it takes but one demon to destroy a city" replied the demon, who then pointed far off into the distance to a black cloud.
"over there is a monastery and circling above is a hundred thousand demons for every man of faith within her walls" the demon continued "where will you rest your head tonight wanderer?"