Many composers have created music for this famous poem by Goethe. I have heard in excess of 40, but I haven't verified it. However, the most successful of them all has to be Franz Schubert.
His version of Erlkönig has been hailed as the beginning of German romanticism in music. It's quite a feat considering he was only 18 at the time.
So what about the music? To start off with it is written for piano and voice in the German "lieder" tradition. It starts out with triplets of octaves in the right hand to illustrate a galloping horse, a pattern that goes through almost all the way to the end, and then an ominous sounding melody in the left hand. (I'd sing it to you if I could but it doesn't really work in plain text.) Then the singer starts. At this point I have to add that Erlkönig is fun to sing, partly because you get to play four different characters, the narrator, the father, the son and the elf king. Anyway, the singer starts with the narrator, who is calm and neutral. The second paragraph brings in the other characters. The father is all through the song attempting to be calm and reassuring while obviously worried for his son. The son gets more and more scared as the song goes on, and I would like to think that he is ill with a high fever. That would explain the father's worry as well as being out late at night. Finally, the elf king starts off being very nice and seductive, but ending up being purely evil.
Although it's a delightful piece of music to listen to, and fun to perform, it is to the performers a bit like a 400 metre race. It's fast, it lasts for 4 minutes (OK, the metaphor is not that good), it is difficult and leaves you gasping for air afterwards.
If you want to listen to a nice interpretation I would recommend that you go to your local library and look for Dietrich Fischer-Dieskau as singer. Any of his recordings will do Erlkönig, and most other Schubert and Schumann lieder, justice.