Halfdan and Aasa had issue Eystein Halfdansson, 725-780.
Jarl of Vestfold, Ringerike, Hadeland, and the Opplands. He married Hild Ericsdottir, 730-790, daughter of Eric Agnarsson.
Eystein and Hild had issue:
1. Siegfried Eysteinsson, 745-798, whom is identified as Sigfridi regis Danorum. [Annales Fuldenses, 782.] The family's half-Danish ancestry seems to be confirmed by Siegfried's accession to the Danish throne.
2. Halfdan Eysteinsson, 746-800, Jarl of Vestfold, Ringerike, Hadeland, and the Opplands. He was born in Vestfold at a place called Holtar, the present Holtan in Borre, and is buried under a mound at Borre. He was known as a great warrior who often pillaged and gathered great booty. His nicknames were Halfdan The Mild, signifying his generous nature, and Halfdan The Bad Entertainer. There seems to be a contradiction here, but I think it is easily explained. He was generous to his men by way of rewarding them with money and land, but, when they were guests at his house, they received rather stingy amounts of food and drink. This could have been due to him encouraging their fitness, or, more likely, that his wife, Hlif Dagsdottir, 748-810, whose name derived from the Old Norse Hilfar, meaning shield, ran an extremely economical household. She was the daughter of Jarl Dag of Vestmar.
3. Harald Eysteinsson, 746-804, killed in a battle in the Irish Sea, who married Imhild Von Engern, 760-812, daughter of Warnechin Graf von Engern and Kunhilde von Rügen. Their issue were: Halfdan Haraldsson, 770-810, who was killed in the battle of Walcheren. Harald Haraldsson, who was murdered in 804. Holger Haraldsson, who died in battle in 807.
Halfdan Haraldsson's children were:
Hemming Halfdansson, who was killed in the Battle of Walcheren, 837.
Harald Halfdansson, nicknamed Klak, meaning complainer, who was was killed in the Battle of Walcheren, 844. He was also known by the appelations of Hericus, Heriold, and Heriolt. His children were: Godfried [de Guines] Haraldsson. Rolf Haraldsson [Annales Bertiniani, 864]. Guthorm Haraldsson, who was killed in battle against Horic I. in 854. Ingebord Haraldsdottir. Thorny Haraldsdottir.
Reginfred Halfdannson, who briefly shared joint regency of Denmark with his brother, Harald. He was killed in battle in 814.
Anulo Halfdansson, killed in battle in 812.
Rorik Halfdansson. He was granted Dorstad by Emperor Lothar in 850, having previously been expelled from this fief. He undertook to protect this part of Frisia from further Viking attack, but lacked the military power to fulfill this obligation. In 857, three years after the accession of Horik II., he gained land around Hedeby, and held most of Northern Frisia. It is often claimed that he was the founder of the Russian State [N. T. Belaiew, Saga-book of Viking Society, x., pt. ii., p. 267, 1925-7].
4. Geva Eysteinsdottir, 749-816, who married Duke Wittikind of Westphalia, principal progenitor of the Dukes of Saxony. Their daughter was Hasala von Wettin, 765-827, who married Duke Bruno II. of Saxony, 756-813. Their son was Duke Bruno III. of Saxony, 780-844, who married Susanna de Montfort-sur-Risle, 790-847. They had issue: Count Ludolf I. of East Saxony, 806-864, who married Oda of Thuringia, 816-869. Their daughter was Luitgarde of Saxony, 851-905, who married King Louis II. of France, 846-879 - son of King Charles II. of France, 823-877, and Ermentrude de Orléans, 823-869 - their son being King Charles III. of France, 879-929.
So do not end your research with the royalty of Europe. All these houses are intermarried from the Pre-Christian Jarls to the lesser houses, and through the blue bloods of Christian Europe.
Source: THE ORIGINS OF THE STANHOPE FAMILY