erlking
In German literature and modern folklore, a malicious elf or spirit who preys on children.
This line, and indeed the whole passage (lines 653-664), allude to the well-known poem by Goethe about the erlking, hoary enchanter of the elf-haunted alderwood, who falls in love with the delicate little boy of a belated traveler.
Where it comes from
Partial calque of German Erlkönig (literally “alder-king”), a mistranslation of Danish ellerkonge (“king of the elves”).
Adapted from the Wiktionary entry erlking (CC BY-SA 4.0).