The "Royal Frankish Annals" (in their various forms, expansions, and offshoots) include accounts of relations with Danes and Viking ravages in Frankish territory. The earliest relevant material relates information about Charlemagne's interactions with the Danes under a King Sigifrid / Sigfred in the context of his moves in Saxony (tr. King): "He stayed there some time and dealt with various matters; among other things, he gave audience and leave to depart to legates from Sigfred, king of the Danes, and to those sent to him, supposedly in the cause of peace, by the khagan and the jugur, princes of the Huns. After the assembly had been concluded and he had recrossed the Rhine into Gaul, Widukind, who had taken refuge with the Northmen, returned to his homeland and stirred up the passions of the Saxons with vain hopes so that they rebelled" (Revised Annals for 782). In later years, the Annals are a major source of information about another king of Danes, Godofrid / Godfred, and his successors.
For the history of these early leaders and their relations with the Frankish kingdom, see D. Melleno, Before They Were Vikings (Diss. Berkeley 2014), pp. 28ff.; S. Coupland, "From Poachers to Gamekeepers: Scandinavian Warlords and Carolingian Kings," Early Medieval Europe 7.1 (1998): 85-114; K. L. Maund, "'A Turmoil of Warring Princes': Political Leadership in Ninth-Century Denmark," The Haskins Society Journal 1994: 29-47.
Cf. also the Annals from the British Isles (Anglo-Saxon Chronicle and Chronicle of Ireland) for roughly similar kinds of sources from other areas.