Border ballads, like all traditional ballads, were traditionally sung unaccompanied. There may be a repeating motif, but there is no "chorus" as in most pop songs. The supernatural is a common theme in Border ballads, as are recountings of raids and battles.
Representative samples include "Thomas the Rhymer" (aka "True Thomas", Thomas of Erceldoune"), which opens in the Scottish town of Erceldoune (modern Earlston, Berwickshire); and the very famous "Tam Lin".
The interested reader is referred to Minstrelsy of the Scottish Border by Sir Walter Scott.
'The bare rolling stretch of country from the North Tyne and Cheviots to the Scottish southern uplands was for a long time the territory of men who spoke English but had the outlook of Afghan tribesmen; they prized a poem almost as much as plunder, and produced such an impressive assembly of local narrative songs that some people used to label all our greater folk poems as 'Border ballads'.' - AL Lloyd, 'Folk Song in England'.p.150