Short answers, pulled from the story.
Hjaðningavíg is a Germanic heroic legend about a never-ending battle. The name translates as "the battle of the Heodenings," referring to the people of Heðinn, one of the central figures in the legend.
Heðinn is a prince and the son of Hjarrandi who kidnaps Hildr. Hǫgni is Hildr's father, who pursues Heðinn and fights him. Hildr stands between them, and each night uses incantations to resurrect the fallen warriors so the battle begins again.
Dáinsleif is the sword Hǫgni draws before the battle begins. It inflicts wounds that never heal and, like the cursed blade Tyrfing, must kill a man every time it is unsheathed. Drawing it made reconciliation impossible.
Snorri Sturluson identifies the island as Hoy in Orkney, in his account of the legend in Skáldskaparmál.
In the older tradition the battle was fated to last until Ragnarok. In Sörla þáttr, a short story in the Flateyjarbok written by two Christian priests in the 15th century, the arrival of Christianity under King Olaf Tryggvason breaks the curse and ends the eternal conflict.
The legend appears in Skáldskaparmál, Ragnarsdrápa, Sörla þáttr, Skíðaríma, and Gesta Danorum, as well as the Old English poems Deor and Widsið. It also survives in the Middle High German Kudrun, the Old Yiddish Dukus Horant, an Anglo-Latin manuscript at Oxford's Bodleian Library (Bodley 614, folio 49v), and the 18th-century Norn ballad "Hildina." An image stone at Stora Hammar on Gotland is also believed to depict the legend.