At that time Norway was very disturbed. Harald Shockhead, the son of Halfdan the Black, till then king of the Upplands, was aiming at the supreme kingship. He went into the North and fought many battles there, in which he was always victorious. Then he marched harrying through the territories to the South, bringing them into subjection wherever he came. On reaching Hordland he was opposed by a motley multitude led by Kjotvi the Wealthy, Thorir Long-chin, and Soti and King Sulki from South Rogaland. Geirmund Swarthyskin was then away in the West, beyond the sea, so he was not present at the battle, although Hordland belonged to his dominion.

Onund and his party had arrived that autumn from the western seas, and when Thorir and Kjotvi heard of their landing they sent envoys to ask for their aid, promising to treat them with honour.

They were very anxious for an opportunity of distinguishing themselves, so they joined Thorir's forces, and declared that they would be in the thickest part of the battle. They met King Harald in a fjord in Rogaland called Hafrsfjord. The forces on each side were very large, and the battle was one of the greatest ever fought in Norway. There are many accounts of it, for one always hears much about those people of whom the saga is told. Troops had come in from all the country around and from other countries as well, besides a multitude of vikings. Onund brought his ship alongside of that of Thorir Long-chin in the very middle of the battle. King Harald made for Thorir's ship, knowing him to be a terrible berserk, and very brave. The fighting was desperate on either side. Then the king ordered his berserks, the men called Wolfskins, forward. No iron could hurt them, and when they charged nothing could withstand them. Thorir defended himself bravely and fell on his ship fighting valiantly. The whole ship from stem to stern was cleared and her fastenings were cut, so that she fell out of the line of battle. Then they attacked Onund's ship, in the forepart of which he was standing and fighting manfully. The king's men said: "He bears himself well in the forecastle. Let us give him something to remind him of having been in the battle." Onund was stepping out with one foot on to the bulwark, and as he was striking they made a thrust at him with a spear; in parrying it he bent backwards, and at that moment a man on the forecastle of the king's ship struck him and took off his leg below the knee, disabling him at a blow. With him fell the greater number of his men. They carried him to a ship belonging to a man named Thrand, a son of Bjorn and brother of Eyvind the Easterner. He was fighting against King Harald, and his ship was lying on the other side of Onund's. Then there was a general flight. Thrand and the rest of the vikings escaped any way they could, and sailed away westwards. They took with them Onund and Balki and Hallvard Sugandi. Onund recovered and went about for the rest of his life with a wooden leg, wherefore he was called Onund Treefoot as long as he lived.