There is a poem by Goethe about the Erlkoenig, which was set to music by Schubert (amongst others). The literal translation is alder-king, rather than elf-king. There are also other versions (song & poem) with the same title, some of which are 'filk'. I can't give you any details. I can only tell you that Rika was practising one some 15 years ago & I found, somewhat to my surprise, that I could read the German lyrics upside down, my only German being long-forgotten (or so I thought) schoolboy learning! I think the only word I couldn't puzzle out was the Erl of Erlkoenig! ;-)