Halfdan Ragnarsson was a real 9th-century Norse military leader, son of the legendary Ragnar Lothbrok, known for co-commanding the Great Heathen Army in England, becoming the first Viking King of York (Jorvik), and fighting in Ireland and Scotland. Historically, he was a pragmatic and powerful commander, leading conquests in Northumbria, East Anglia, and Mercia, distributing land to his followers, and establishing a kingdom, though saga embellishments blend his story with others, making his exact "heroic" portrayal a mix of fact and legend.

Key Historical Aspects:
Great Heathen Army: Co-led the invasion of Anglo-Saxon England starting in 865, a pivotal event in Viking history.
King of Jorvik: Established himself as the first Viking king of the conquered Northumbrian city of York, minting coins there.
Conqueror & Administrator: Subdued Northumbria, campaigned against Picts and Strathclyde Britons, and distributed conquered lands.
Irish Campaigns: Fought for control of Dublin and was potentially killed in Ireland around 877, possibly as "Albann".

In Summary: Halfdan was a significant historical figure, a cunning leader and conqueror who shaped Viking Britain, though the "heroic" narrative surrounding him is often amplified and merged with tales of his legendary family, making him a complex character of both history and folklore.