The Demon Lover, also known as the House Carpenter.
Two good versions I can think of are Steeleye Span and The iron Horse. I think the latter is better. Oh and Dylan has one too.
Basicaly, and I'm it's about a woman who was engaged to a man gone to sea, but has long since married another. One day her love returns and compells her to go with him. When at sea then she finds out he has come to drag her down to Hell. Her lover is either a ghost or the Devil himself, depending on which version you follow.
In the House Carpenter versions (very popular in America), the supernatural elements are gone, and instead it is merely a tale of divine punishment for adulterous lovers.
Sweeney's Men does this one.
The Wife of Usher's Well is a folksong equivalent of the Monkey's Paw. A widow sends her three sons off to sea, who then die. She is wracked with grief and refuses to accept the fact that they are gone, calling them to return. They do so, a year(?) later, and she invites them to eat and sleep, but they say they cannot remain past dawn. When the cocks cry, they dissapear.
Martin Carthy's version (with his daughter on fiddle) is set to a Basque tune. Stunningly beautiful.
If you have done the Erlking, why not contrast it with The False Knight on the Road? The Devil tries to trick a young boy, but fails.