Dauferio was born in 1026. He was the only child of Prince Landulf V of Benevento, one of the last Lombard rulers in Italy. After his father died in battle with the invading Normans in 1047, Dauferio fled from an arranged marriage and, though brought back by force, eventually fled again. He went to Cava de' Tirreni, where he obtained permission to enter the monastery of S. Sophia at Benevento, where he changed his name from Dauferius to Desiderius. It was a decision that his mother vehemently opposed, as he was the sole heir.