I read that in elementary school, sometime between 2nd grade (age 7) and 5th grade (age 10) in a textbook and was haunted by it for many years. He's not just the Erl (elf) King, he's the Boogeyman! Who would put such a scary poem in an elementary schoolbook? (Someone who hates children, I'm sure.) Think about how horrifying it is from a kid's point of view: only the kid can see the Erl-King/Boogeyman and he kills the kid at the end! What a ghost story!
I hope you enjoy this scary little poem.
Have fun, safe, and happy Halloween!