Learn the origins of the Brotherhood from this excerpt of Ari’s saga, a record he keeps through the centuries, and which is continued at the beginning of each Immortal Brotherhood book…
In the early years of the raids upon Britain, it came about that Håkon IronToe, a high chieftain of the Norse, heard tales told among his Saxon thralls that the men of Odinsbrigga, in the Kingdom of Anglia, guarded a great treasure. Determined to have it, he sent his fiercest warriors, led by Brand Einarsson, called Thor’s Hammer, to take the village and bring the gold to him. But the treasure was protected not only by the swords of men, but by the sorcery of the witch, Cwen, who conjured warriors from her own blood and sent her son to lead them.
When Brand saw his men being slaughtered, a great rage came over him, and he gained the strength of ten berserkers. He set upon the ghosts, slashing and hewing until his blade found solid flesh, and he did kill the son of Cwen.
In the fury of her grief, Cwen used her magic to bind Brand and those of his men whose hearts still beat, and she had them carried before the treasure they had come to take, and she cursed them. She turned them into shadow beasts, living half as animal, half as man, each taking the form of his fylgja, the spirit companion whose image he wore on a chain. When she had done, Cwen took their amulets and scattered them across the face of the earth, and she drove the men off into the forest to be hunted.
When word of Brand’s fate reached Håkon, he trembled in fear and ordered his boats to sail, but a great wave arose and his ship vanished from the face of the sea. He never knew of the greater curse that befell his men, for Cwen also made the warriors immortal, so that their torment should go on and on. Forever.
After a time, the men of Odinsbrigga lost interest in their sport, and Brand searched out his men to gather them together. But those who were beasts set upon those who were men, and were set upon in turn when the sun fell or rose, and the vileness of the magic protected them from death but not from pain. When it grew clear they could not live together thus, each warrior set off to make his own way. Before they scattered, Brand swore a vow to every man that he would hunt Cwen until he found her, and that he would make her pay for what she had done.
The years passed into centuries, and still Brand hunted. One by one, his warriors learned to live among men once more. The first of these was Ivar, son of Thorli, called Greycloak, who spent his days in the form of an eagle . . .
—from the Dyrrekkr Saga of Ari Sturlusson
(E.L. Branson, trans.)
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*Ivar’s story  is  told in Immortal Warrior, where you’ll also meet Ari and Brand.