Haakon Sigurdsson lost no time in proclaiming himself king of all Norway, though he dispensed preliminarily with the ceremony of a formal proclamation at Oere-thing. As he was but a boy of fourteen, it was, of course, his advisers who dictated his actions. He was a tool in the hands of a few ambitious liegemen, who had staked their fortunes on the chance of his ascendancy. For the purpose of portioning out lucrative offices among his adherents, he called a secret meeting in the church of St. Hallvard in Oslo. As it was of great importance to Erling Skakke to know how these men felt toward him, his wife Christina bribed the priest who kept the keys of the church, to conceal one of her friends where he could hear the proceedings. She then sent a messenger to her husband, enjoining him under no circumstances to trust Haakon or his party. But Erling was too conspicuous a man to be allowed to preserve neutrality; and as he could not declare for Haakon, he was obliged to declare against him. He then proposed, though scarcely in good faith, to make the child, Nicholas Simonsson, the son of Simon Skaalp and Harold Gille's daughter Maria, the head of the opposition and to proclaim him king. There were, however, serious objections to this course; and after many consultations, Erling allowed himself to be urged to do, what had been his intention from the beginning, viz., to proclaim his own son Magnus. Magnus, to be sure, was not of royal birth, but he had royal blood in his veins, being the grandson of Sigurd the Crusader. By shrewd manoeuvres, Erling succeeded in rallying the greater part of King Inge's party about his son, who was but five years old, and to induce the liegemen to swear him allegiance. A thing was then summoned to meet in Bergen, and Magnus was formally acknowledged as king (1161).

The land was now divided between two tolerably evenly balanced parties, and only the sword could decide to which of the two the government should belong. After the great defeat at Oslo, however, Inge's party had lost much of its prestige, and Erling felt that foreign help was needed to secure its predominance. He, therefore, sailed with his son and a large following of high-born men to Denmark and obtained the promise of help from King Valdemar the Great on condition of ceding to him the greater part of Viken. Haakon availed himself of his absence to take possession of the fleet which had belonged to Inge and to have himself proclaimed king at Oere-thing. His friend Sigurd of Reyr he raised to the dignity of earl, and delegated to him the task of watching for Erling, whose return from Denmark was expected. Erling was, however, a shrewd man who did what was least expected of him. He did, indeed, return from Denmark, but by a singular route. He crossed from Skagen in Jutland to Agder, and thence steered northward to Bergen, where he killed or otherwise punished many who had given their allegiance to Haakon. Then, before Earl Sigurd had yet heard of his arrival, he attacked Haakon in Tunsberg and beat him. Having accomplished this and secured the recognition of his son in Viken, Agder, Rogaland, and HÖrdaland, he returned to Bergen where he spent the winter. Haakon, who found his strongest support in TrÖndelag, went into winter quarters in Nidaros.

It was merely a question of time when the two rival kings, or the men who represented them, should meet for a final contest. Therefore, as soon as the ice broke up, preparations began on a grand scale. Erling's cunning again stood him in good stead, for by a stratagem he succeeded in surprising Haakon at Sekken, in SÖndmÖre, and utterly destroying him (1162). The poor boy, who was but fifteen years old, jumped from his ship on board the one which lay nearest, and found himself unexpectedly among enemies. He told the men who he was, and surrendered himself to their mercy. The battle was then virtually at an end; but when Erling found that the men to whom Haakon had surrendered were determined to guard his life, he began a fresh attack, and managed it so that, in the tumult, the boy-king was slain. His own former candidate for the throne, Nicholas Simonsson, whom he had forcibly abducted from Bergen, he also contrived to get rid of in the same battle, and there can be little doubt that he was responsible for his death. Haakon Sigurdsson was king of Norway for about a year and three months. He was large for his age, and on account of the slenderness of his waist and the breadth of his shoulders, was called Haakon the Broad-Shouldered (Herdebred).