He was presumably king of a stronger and more unified Denmark that rose at the end of the 7th century.
Ongendus' reign probably followed the rise of a strong kingdom in central Jutland which lasted from about 200 to 600 CE, and from 400 also included Kent and Isle of Wight.
[3] 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 savagery of Ongendus may well be overstated.