Halfdan Ragnarsson was a Viking leader and a son of the legendary warrior Ragnar Lothbrok. He played a significant role in the Viking Age during the late 9th century, particularly in England and Ireland. Halfdan is known for his military campaigns and for being one of the leaders of the Great Heathen Army, which invaded England in 865 AD. Halfdan is often associated with the establishment of the Viking kingdom in York, where he ruled as a king. His legacy includes the spread of Norse culture and influence in the British Isles, as well as his contributions to the Viking raids that shaped the history of the region.