Morgan le Fay, Morgaine la Fee
or Morgana the Fairy. Daughter of Queen Igrayne, and half-sister of King Arthur, who revealed to him the intrigues of Sir Lancelot and Guinever.
She gave him a cup containing a magic draught, and Arthur had no sooner drunk it than his eyes were opened to the perfidy of his wife and friend.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894