In mainly Teutonic and Norse folklore, the elves were originally the spirits of the dead who brought fertility. Later they became supernatural beings, shaped as humans, who are either very beautiful (elves of light) or extremely ugly (dark / black elves). They were worshipped in trees, mountains and waterfalls. The Danish elves are beautiful creatures, but they have hollow backs. The Celtic elves are the size of humans.
The belief in elves, or supernatural and invisible beings, is almost universal. Apparently, there has been no primitive tribe or race that has not believed at one time or another that the world was inhabited by invisible beings. Especially on the British Isles the belief was very profound. In stories from the 8th and 9th century there are many references to elves, or fairies as they are called there. The king of the elves, Oberon, and his wife Titania appear in some very important works of medieval literature, such as Huon de Bordeaux and Shakespeare's A Midsummer Night's Dream.