He was presumably king of a stronger and more unified Denmark that rose at the end of the 7th century, building Danevirke.
Given the time we must assume that Ongendus was involved with the construction of the Danevirke, as it was under construction at this time.
About 710, Saint Willibrord visited the Danes whilst Ongendus was ruling and returned with 30 boys to instruct in missionary work. No further details are given about Ongendus, other than that he was "more savage than any beast and harder than stone" — the ideal of man in the Viking Age. Against Willibrord's account, however, one should also consider that he was apparently well received, could travel in peace through Ongendus' realm and was allowed to return with his potential disciples, so the savagry of Ongendus may well be overstated. It may just have been the obligatory classification of any heathen ruler.
- Alcuin's Life of St. Willibrord, translated in C. H. Talbot, "The Anglo-Saxon Missionaries in Germany" (London and New York, 1954), especially pp. 9–10.
- Voss, Olfert (2011), "Skalk 2011:6", Jernstudier (in Danish), pp. 18–23.
- Myhre, Bjørn (2003), "The Iron Age", The Cambridge History of Scandinavia, p. 87., ISBN 0-521-47299-7
- Skovgaard-Petersen, Inge (2003), "The making of the Danish kingdom", The Cambridge History of Scandinavia, p. 172., ISBN 0-521-47299-7