Keats' literary ballad "La belle Dame sans Merci" (the beautiful pitiless lady) deals with the plight of a "knight-at-arms" who has been seduced by "a full beautifull faery's child" only to be deceived and enslaved by her.
The poem begins with the narrator asking the knight why he is wandering all alone on the bleak countryside at this odd time of the year. The pale and haggard knight replies that he met a beautiful, ethereal lady with whom he fell in love with straightaway. She reciprocated his offerings of love tokens and soon immediately took him to her home, where they consummated their love.
Soon she charmed him to sleep and vanished. While asleep he had a horrifying dream in which mighty kings and warriors-the former victims of this beautiful maiden-declared that the beautiful pitiless maiden had enslaved him forever. Frightened, he awakens from his nightmare to find himself wandering all alone on this deserted stretch of the countryside hoping that death would soon put him out of his misery.
Keats was suffering from T.B. and he knew that he will die soon, so he was depressed. The overall mood of the poem reflects this disconsolate state. The expression "pale and haggard" describes the physical state of a person suffering from T.B.