What is "hildina"

"Hildina" is a traditional ballad thought to have been composed in Orkney in the 17th century, but collected on the island of Foula in Shetland in 1774, and first published in 1805. It tells a story of love, bloodshed and revenge among characters from the ruling families of Orkney and Norway. This ballad is written in Norn, the extinct North Germanic language once spoken in Orkney and Shetland, and is the only surviving work of any length in that language.