The Erlkönig (Erlking) is depicted in a number of German poems and ballads as a malevolent creature who haunts forests and carries off travellers to their deaths. The name may be an 18th-century mistranslation of the original Danish word elverkonge, "elf-king". The character is most famous as the antagonist in Goethe's poem Der Erlkönig and Schubert's musical adaptation of the same name.
The Erlking as a character has its origins in a common European folkloric archetype, the seductive but deadly fairy or siren (compare La Belle Dame sans Merci and the nix). In its original form in Scandinavian folklore, the character was a female spirit, the elf-king's daughter (Elverkongens datter). Similar stories existed in numerous ballads throughout Scandinavia in which an elverpige (female elf) was responsible for ensnaring human beings to satisfy her desire, jealousy or lust for revenge.
The Erlking's Daughter
Johann Gottfried von Herder introduced this character into German literature in Erlkönigs Tochter, a ballad published in his 1778 volume Stimmen der Völker in Liedern. It was based on the Danish folk ballad Hr. Oluf han rider "Sir Oluf he rides" published in the 1739 Danske Kæmpeviser. Herder undertook a free translation where he translated the Danish elvermø ("elf maid") as Erlkönigs Tochter; according to Danish legend old burial mounds are the residence of the elverkonge, dialectically elle(r)konge, the latter has later been misunderstood in Denmark by some antiquarians as "alder king", cf Danish elletræ "alder tree". It has generally been assumed that the mistranslation was the result of error, but it has also been suggested (Herder does actually also refer to elfs in his translation) that he was imaginatively trying to identify the malevolent sprite of the original tale with a woodland demon (hence the alder king).
The story portrays Sir Oluf riding to his marriage but being entranced by the music of the elves. An elf maiden, in Herder's translation the Elverkonge's daughter, appears and invites him to dance with her. He refuses and spurns her offers of gifts and gold. Angered, she strikes him and sends him on his way, deathly pale. The following morning, on the day of his wedding, his bride finds him lying dead under his scarlet cloak.
Although inspired by Herder's ballad, Goethe departed significantly from both Herder's rendering of the Erlking and the Scandinavian original. The antagonist in Goethe's Der Erlkönig is, as the title suggests, the Erlking himself rather than his daughter. Goethe's Erlking differs in other ways as well: his version preys on children, rather than adults of the opposite sex, and the Erlking's motives are never made clear. Goethe's Erlking is much more akin to the Germanic portrayal of elves and valkyries - a force of death rather than simply a magical spirit.
In later English literature
Lord Dunsany's novel The King of Elfland's Daughter (1924) draws heavily on the ideas established by Herder's poem.
In Angela Carter's short story 'The Erl-King', contained within the 1979 collection The Bloody Chamber, the female protagonist encounters a male forest spirit. She is seduced by him before discovering his malicious intentions, and finally manages to escape.
In Jim Butcher's Dresden Files mythos, the Erlking is a Hunter Spirit, King of the Goblins, and a powerful Wyldfae (A fairy not bound the Summer or Winter courts), and one of the few beings able to summon the Wild Hunt. He is portrayed as a towering figure wearing a horned helm. While considered alien, powerful, and dangerous, he is not evil so much as neutral to the common morality of mankind, living only for the Hunt.