The Collaborative International Dictionary of English v.0.48:

Erlking \Erl"king`\, n. [G. erlk["o]nig, fr. Dan. ellekonge
   elfking.]
   A personification, in German and Scandinavian mythology, of a
   spirit or natural power supposed to work mischief and ruin,
   esp. to children.
   [1913 Webster]