The joke is a pun on the word 'raven'. Edgar Allen Poe has a famous poem with a raven.  'Raven' sounds like 'raving' which would make the phrase 'raven madman' into 'raving madman'.  A 'raving madman' is someone who is wildly eccentric and 'crazy'.
Many people think that Edgar Allan Poe was a raven madman .