Short answers, pulled from the story.
The legend of Hjaðningavíg appears in the 13th-century text Skáldskaparmál written by Snorri Sturluson. It also surfaces in Ragnarsdrápa a poem attributed to Bragi Boddason from the early 9th century.
The core characters include Heðinn Högni and their daughter Hildr who stood alone on the battlefield after sunset. Hildr held incantations that caused fallen soldiers to rise again with weapons drawn each dawn.
King Olaf I of Norway arrived on the scene during the late 10th century when he was the first to encourage Christianity in Norway. His presence dissolved the ancient curse that bound the warriors to eternal combat.
Archaeologists date the Stora Hammars I stone on the island of Gotland off Sweden's coast to between the 7th and 8th centuries CE. Another stone at Smiss (I) depicts similar scenes of endless warfare from the same period.
The Old English poem Deor mentions the Heodenings in its opening lines where a poet named Deor wrote about serving them for many years. Modern scholars interpret this text as irony since the Heodenings never truly ended their war.