In Norse mythology, Forseti is the son of Baldr and Nanna, and has his residence and estate in Asgard. He is mentioned by Snorre in the edicts of Grimnismal, which describe his hall, Glitner, with a roof of silver and golden posts, where he makes peace and reconciles. Forseti is the fairest arbiter of gods and men.
Forseti was probably originally neither son of Baldr nor judge of conciliation. When St Willibrord came to Frisia in the 700s, he heard of a pagan idol, Fosite, on an island between the Frisian and Danish lands; a shrine where Willibrord went ashore and slaughtered the island's sacred cattle. He used the water from the holy spring to baptize in.
Probably this Frisian Fosite was brought to Norway, perhaps by Frisian merchants. What his Frisian name referred to, and how widespread the worship of him was among the Frisians, is not known; but he was no mean god when he was given a place in the Norse circle of gods.
According to legend, twelve Asegeir ("elders") once wandered throughout Scandinavia assimilating local laws. They wanted to take the best laws from each tribe and write more applicable ones for all of them to live together in peace.
It is said that after collecting the various laws from all regions, they embarked by sea to a distant place where they could discuss the writing calmly. However, a dangerous storm arose and they were thus left at the mercy of the waves.
Then they invoked the name of Forseti to protect them from death, and then they saw a thirteenth person appear on the boat, guiding it to a deserted island in safety.
There this person, presumably Forseti himself, divided the land and created a spring. After consecrating the place, he wrote a code of laws that combined the best of the local ones and suddenly disappeared.
The island became a place of worship of Forseti and the spring its sacred place. The integrity of the island was respected by all Norse and no warrior ever raided the island for fear of being transported to Hel instead of Valhalla.
All major assemblies were held on that island. Disputes were settled in daylight and never in winter, because it was believed that darkness made a kind of anathema to the truth, preventing disputes from being resolved properly.
"Fositesland" is mentioned by Alcuin as in confinio Fresorum et Danorum. Adam of Bremen identifies "the island of Forseti" with Heligoland.