Magnus Erlendsson, born in 1075, is the son of Erlend Thorfinnsson, Earl of Orkney, and had first served Magnus III of Norway as skutilsvein (approx. Chamberlain), who took possession of the islands in 1098, deposing Erlend and his brother, Paul.

Paul's son, Haakon Paulsson, then became regent on behalf of the Norwegian prince, Sigurd, who in 1105 had made Haakon earl.

Magnus according to the Orkneyinga Saga had a reputation for piety and gentleness, and was rejected by the Norwegians, refusing to fight in a Viking raid in Anglesey, Wales, because of his religious convictions, instead staying on board his ships during the Battle of Menai Strait, singing psalms.

He had been obliged to take refuge in Scotland, but returned to Orkney in 1105 and disputed the succession with his cousin Haakon Paulson.

Having failed to reach an agreement, he had sought help from King Eystein I of Norway, who grants him the earldom of Orkney in 1108; he rules jointly and amicably with Haakon.

Haakon and Magnus have a falling-out in 1114 and the two sides meet at the Thing (assembly) on the Orkney mainland, ready to do battle.

Peace is negotiated and the Earls arrange to meet each other on the island of Egilsay, each bringing only two ships.

Magnus arrives with his two ships, but then Haakon treacherously turns up with eight ships.

Magnus takes refuge in the island's church overnight, but the following day he is captured and offers to go into exile or prison, but an assembly of chieftains insists that one earl must die.

Haakon's standard bearer, Ofeigr, refuses to execute Magnus, and an angry Haakon makes his cook Lifolf kill Magnus by striking him on the head with an ax.

It is said that Magnus first prayed for the souls of his executioners.

According to the sagas, the martyrdom took place after Easter, on April 16.

The year is often given as 1115, but this is impossible, as April 16 fell before Easter that year.