One of the many minor characters who doesn't appear, but still has an influence on the plot, in the Discworld
Achmed The Mad is the author of the infamous Necrotelicomnicon or Liber Paginarum Fulvinarum (Latatian for Book of Yellow Pages). Achmed preferred to be known as Achmed The I Just Get These Headaches.
Achmed is supposed to have written this book after drinking too much Klatchian coffee and becoming knurd - the state beyond sobriety in which you see the world as it truly is.
Little is known of Achmed, as shortly after his death the 'about the author' page in the one surviving copy of the Necrotelicomnicon (in the library at Unseen University) underwent spontaneous combustion. However the 'other titles by this author' page survived and lists one other book - Achmed The I Just Get These Headaches' Book Of Humorous Cat Stories. Many people think this explains a lot.