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In page Harald Bluetooth:

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As noted above, Harald's father, Gorm the Old, had died in 958, and had been buried in a mound with many goods, after the pagan practice.[citation needed] The mound itself was from c. 500 BCE, but Harald had it built higher over his father's grave, and added a second mound to the south. Mound-building was a newly revived custom in the 10th century, perceivably as an "appeal to old traditions in the face of Christian customs spreading from Denmark's southern neighbors, the Germans".[9]