Ethelred the Pious
Hrolfr was a Danish chieftain and a veteran of great raids in France and Ireland. In 895 he settled in England with his wife Poppa, their infant son Vilhjálmr, and a large band of followers. King Hogni of Jorvik let them settle in Devon, at that time a frontier region of Jorvik that was almost 100% Anglo-Saxon. The followers that Hrolfr brought to his new jarldom were a major step in the Norsification of the south.
A restless warrior, Hrolfr led an army into the still independent kingdom of Kernow (Cornwall) in 904. He secured the kingdom's submission and placed his 11-year-old son on the throne. Vilhalmer/William spent most of his time in Kernow and gradually lost control of the jarldom in Devon. In 924, King Hogni died, and a number of rival jarls contended for the throne of Jorvik. Hrolfr and Vilhalmer's candidate lost to the new king, Bjorn I, and this further hurt their position in England.
Hrolfr died in 931. In 946, the Viking conqueror Erik the Mariner came to Kernow with a number of fighters, and Vilhalmer had little choice but to submit to his overlordship. He remained an independent monarch in practice, however. Vilhalmer died in 960. Vilhalmer's son Donyarth allowed Erik to use Kernow as a base for attacking Ireland. After the Bloodaxe War finally ended, Kernow was largely forgotten by the major Viking powers
With a new dynasty of feuding overlords to deal with, Donyarth had little attention to spend on his Jorvikish domains. He failed to reconquer the Devon jarldom, and thenceforward Hrolfr's descendants ruled Cornwall as a fully independent Norse-Celtic kingdom. They maintained its independence well into the 11th century.