Inge is the only legitimate son of the murdered king Harald Gille by his wife, Ingiríðr Ragnvaldsdóttir.

At this time, however, legitimate birth is not an important factor in determining succession to the Norwegian throne.

Inge is fostered by Ögmund or Ámund Gyrðarson in eastern Norway.

The one-year-old Inge is named king at the thing of Borgarting near Sarpsborg.

His two half-brothers, also infants, Magnus and Sigurd, are also named king at other things.

Their respective guardians join forces against Sigurd Slembe and his ally, the former king Magnus the Blind.

Magnus had rejoined Sigurd Slembe's men, but they continue to have little support in Norway.

After some time spent more like bandits than kings, they meet the forces of the child kings King Inge I and King Sigurd II in the naval Battle of Holmengrå (Slaget ved Holmengrå) near Hvaler in Oslofjord.

The loyal guard Reidar Grjotgardsson lifts his crippled king at the final battle, but a spear impales them both.

Sigurd Slembe, captured and betrayed, meets his end in a brutal manner, tortured to death.

His arms and calves are first crushed with ax-hammers, after which the skin on his head is cleaved, his back is flayed and his spine broken.

He is thereafter hanged, decapitated and thrown into a scree of rocks.

The Norwegian nobility (the lendmenn) have cooperated during the minority of the brothers, Sigurd, Inge and Magnus, sons of Harald Gille, king of Norway, to rule the kingdom and advise the kings.

Their half-brother Eystein had in 1142 come to Norway from Scotland.

His parentage had been accepted, since Harald Gille had acknowledged that he had a son overseas.

Eystein had thus become king and co-ruler together with Sigurd and Inge.

Magnus, of whom little more is known, had at some point in the 1140s died of natural causes.

As they grew up, and their old advisors died, hostility had begun to grow among the three surviving half-brothers.

All three of them are set to meet in Bergen in 1155 in an effort to keep the peace.

Inge accuses Sigurd and Eystein of planning to have him dethroned.

Sigurd denies the accusations, but a few days later one of Inge's guards is killed by one of Sigurd's.

At the advice of his mother Ingrid and his senior advisor, Gregorius Dagsson, Inge orders his men to assault the house where Sigurd is residing.

The King has but few men, and no mercy is given, and on February 6, 1155, Sigurd falls.

He is buried by the old cathedral of Bergen, in what is today Bergenhus Fortress.

(This cathedral will be demolished and replaced by a larger cathedral soon after.)

King Eystein is late in arriving for the meeting, and only approaches the city after Sigurd is already dead.

An uneasy settlement is reached between Inge and Eystein, but peace between them will not last long.

As it turns out, the killing of king Sigurd has started the second phase of the Norwegian civil war era, with fighting continuing until 1208 with only short breaks.