There was a man called Eystein, who gave himself out for a son of King Eystein Haraldson. He was at this time young, and not full grown. It is told of him that he one summer appeared in Svithjod, and went to Earl Birger Brosa, who was then married to Brigida, Eystein's aunt, a daughter of King Harald Gille. Eystein explained his business to him, and asked their assistance.
Both Earl Birger and his wife listened to him in a friendly way, and promised him their confidence, and he stayed with them a while. Earl Birger gave him some assistance of men, and a good sum for travelling expenses; and both promised him their friendship on his taking leave. Thereafter Eystein proceeded north into Norway (A.D. 1174), and when he came down to Viken people flocked to him in crowds; and Eystein was there proclaimed king, and he remained in Viken in winter.
As they were very poor in money, they robbed all around, wherefore the lendermen and bondes raised men against them; and being thus overpowered by numbers, they fled away to the forests and deserted hill grounds, where they lived for a long time. Their clothes being worn out, they wound the bark of the birch-tree about their legs, and thus were called by the bondes Birkebeins. They often rushed down upon the settled districts, pushed on here or there, and made an assault where they did not find many people to oppose them.
They had several battles with the bondes with various success; and the Birkebeins held three battles in regular array, and gained the victory in them all. At Krokaskog they had nearly made an unlucky expedition, for a great number of bondes and men-at-arms were assembled there against them; but the Birkebeins felled brushwood across the roads, and retired into the forest. They were two years (A.D. 1175-1176) in Viken before they showed themselves in the northern parts of the country.