The Story of Norway by Hjalmar Hjorth Boysen

Erling Skakke had effectually cleared the way to the throne for his son, by killing every descendant of the royal house whom he could lay hands on. There was, however, another undoubted son of Sigurd Mouth left, whom he had not got into his power, besides several whose claims had not yet been pronounced upon. The bitterness between the party of Erling and that of Haakon was indeed so great, that a reconciliation was not to be thought of, and the latter, therefore, seized the opportunity to rally about a king whose royal descent was unquestioned. This new claimant was a boy named Sigurd Marcusfostre (the foster-son of Marcus), probably ten or twelve years old, who had been brought up by Marcus of Skog, a friend and kinsman of Earl Sigurd of Reyr. Another magnate, the much-travelled Eindride the Young, transferred his allegiance to Sigurd, and a large number of proud and adventurous men, who could not tolerate Erling's supremacy, joined the new party.

The peasants, however, who had hitherto suffered but little from the feuds of the kings, now began to find these roving bands troublesome, especially when they levied contributions and foraged wherever they went. Erling availed himself of this circumstance to excite their indignation against the "Sigurd party," as it was called, and he presently succeeded in forcing the hostile chieftains to give battle at Ree, near Tunsberg. Here Earl Sigurd fell, and Eindride the Young, Marcus of Skog, and the boy-king Sigurd were captured and executed.