The Hill-people (Högfolk) is an intermediate class of elves who are believed to dwell in caves and small hills.
Appearance
When Högfolk show themselves they have a handsome human form. The common people seem to connect with them a deep feeling of melancholy, as if bewailing a halfquenched hope of redemption.
Behavior
There are only a few old persons now who can tell any thing more about them than of the sweet singing that may occasionally on summer nights be heard out of their hills, when one stands still and listens, or, as it is expressed in the ballads, “lays his ear to the Elve-hill” (lägger sit öra till Elfvehögg): but no one must be so cruel as, by the slightest word, to destroy their hopes of salvation, for then the spritely music will be turned into weeping and lamentation.
Source
THE FAIRY MYTHOLOGY BY THOMAS KEIGHTLEY (1850)