Who was the Erlking?
The tale of the Erlking follows a common motif in Germanic folklore; a forest-dwelling evil creature, ensnaring human victims. Unfortunately, neither Johann Gottfried Herder's Erl King's Daughter nor Johann Wolfgang von Goethe's Erlkönig includes details on the Erlking's realm or attempt to explore the motivations or purpose of the creature.
What both poems have in common is the creature's method of luring its victims; the promise of gifts. Further, the stories take place in or very near a forest, and the victim doesn't survive in either one. And, both poems tell us of the Erlking's daughters, although Goethe reduces them from main characters in Herder's version to a single reference.
I think that's all the details we can extract from the poems.