Wikipedia
Yngwin ("Yngve") was, according to Gesta Danorum, a king in Götaland, who was a close friend to one of the Danish kings named Halfdan.
This Danish king had no sons, so he left his treasures and his kingdom to Yngwin. Yngwin then moved to Denmark and ruled the kingdom for a while, until he was slain by a competitor to the throne by the name Ragnald.
Yngwin had a son named Siwald, who became king in Denmark after him. Then Siwald's son Sigar took the throne after him. He had three sons.
One of his sons was Alf, who was a Viking. On a raiding tour to Finland, he met Alfhild, the daughter of king Siward in Götaland. Alfhild had her own fleet of Viking ships, some of them manned with maidens. Alf and Alfhild later married, and they had a daughter named Gurid.
In a war fought against a revolting Danish Viking clan, Sigar, Alf and his brothers were killed, while defending their dear Danish subjects. In the end of the war, Alf's comrade Borgar arrived with fresh chivalry from Scania, and slew all the enemies. But without a king, the country now fell apart, and chieftains took control of the different parts.
Only Alf's daughter Gurid had survived of the royal family. She later married one of Borgar's sons, Halfdan, and they had a son named Harald who became the new king. Harald restored the Danish kingdom to its former glory and unity.