Full many knights adventurous and stout
  Have enterprised that Monster to subdue;
  From every coast that heaven walks about,
  Have thither come the noble Martial crew,
  That famous hard achievements still pursue,
  Yet never any could that garland win,
  But all still shrunk, and still he greater grew:
  All they for want of faith, or guilt of sin,
The piteous prey of his fierce cruelty have been.

Book I, Canto VII