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Granmar was married to Hilda, the daughter of the Geatish king Högne of East Götaland, and his son-in-law was the seaking Hjörvard of the Ylfings. These three kings defended themselves against the Swedish king Ingjald Ill-ruler.
According to the Heimskringla, he had no sons, but the legends of Helgi Hundingsbane relate that he had the sons Hothbrodd, Gudmund and Starkad who were slain by Helgi. (Possibly the legend of Helgi originally referred to a different King Granmar, as it seems to be set in the fifth century and Ingjald and the rest are clearly seventh-century.)
Snorri relates that when the Swedish king Ingjald ill-ruler invited seven petty kings in order to burn them all to death inside his hall, Granmar did not arrive and so he escaped being murdered.
The same summer, the Sea-king Hjörvard of the Ylfings arrived at Myrkva Fjord (Mörköfjärden, an inlet that is still used to pass between Mälaren and the Baltic Sea and divides Södermanland in two parts). Hjörvard was invited to a feast with Granmar and he agreed as he had never pillaged in the area before. He was gladly welcomed and the kings sat with one woman each as was the custom, while the others formed a single group. Hjörvard's Vikings did not like that as it was their law that everyone at a feast should drink together. Hjörvard then sat in a high seat opposing Granmar and his men sat on the same bench as him.
Granmar's daughter Hildigunn, who was very beautiful, was told by her father to serve beer to the men. She took a silver goblet, filled it and bowed to Hjörvard, telling him success to all Ylfings, this cup is to the memory of Hrólf Kraki drank half of it and offered the rest to Hjörvard. He drank it, took her hand and asked her to sit beside him. She answered that it was not Viking custom, but he asked her not to mind. This evening they drank and talked a great deal with each other, and the next day, as he was leaving, Hjörvard asked for her hand. Granmar asked his wife, Queen Hilda and the people he respected, telling them to trust Hjörvard. They all agreed. There was a wedding and as Granmar had no sons, Hjörvard stayed to help Granmar defend his kingdom.
In the autumn, Ingjald assembled his forces in order to attack Södermanland. However, when Granmar and Hjörvard heard this they did the same thing and allied with Granmar's father-in-law Högne, the king of East Götaland, and his son Hildur.
Ingjald invaded Södermanland with superior forces, and a battle commenced. After some ferocious fighting, the chiefs of Fjärdhundraland, West Götaland, Nerike and Attundaland began to flee from the battle in their ships. Ingjald was severely wounded and Svipdag the Blind and his sons Gautvid and Hylvid fell. Ingjald retreated and returned to Uppsala and realised that the forces he had gained from the conquered petty kingdoms were not faithful to him.
After a long standstill, some friends of both sides wanted to bring about a reconciliation. Ingjald had a meeting with Granmar and Hjörvard and they decided to have peace for as long as the three men lived. The following spring, Granmar was travelling to Uppsala to confirm the peace between him and Ingjald, but he realised that Ingjald had not promised him a long life and so he returned.
The next autumn, King Granmar and Hjörvard were at a feast in one of their farms on the island Selaön. During the night, they were surrounded by Ingjald and his men and the hall was set on fire burning everyone inside to death. Ingjald then installed chiefs to rule Södermanland.
King Högne of East Götaland and his son Hildur often made raids into the Swedish provinces killing many of Ingjald's men. Högne defended his kingdom successfully until he died.