…Orkney have seen a significant influx of Norwegian settlers during the late eighth and early ninth centuries.

Vikings have made the islands the headquarters of their pirate expeditions carried out against Norway and the coasts of mainland Scotland.

In response, Norwegian king Harald Hårfagre ("Harald Fair Hair") in 875 annexes the Northern Isles (comprising Orkney and Shetland).

Rognvald Eysteinsson receives Orkney and Shetland from Harald as an earldom as reparation for the death of his son in battle in Scotland, and then passed the earldom on to his brother Sigurd the Mighty.

The information we have about the historical Olaf Tryggvason is sparse.

He is mentioned in some contemporary English sources and some skaldic poems.

The oldest narrative source mentioning him briefly is Adam of Bremen's Gesta Hammaburgensis ecclesiae pontificum from around 1070.

Two Latin sagas of Olaf Tryggvason will be written in the 1190s in Iceland Oddr Snorrason and Gunnlaugr Leifsson.

Snorri Sturluson gives an extensive account of Olaf in Heimskringla of about 1230, using Oddr Snorrason's saga as his main source.

The accuracy of these late sources is not taken at face value by modern historians and their validity is a topic of some debate.

According to the sources, Olaf had fought at an early age in Viking battles in Russia, Denmark, and the Netherlands, then joined in raids on the English coast.

Olaf attacks the Orkney Islands in 994, forcibly converting the Orcadian Vikings to the Christian faith.

Thorfinn had ruled alone in Orkney until the return in in about 1037 of his nephew Rognvald Brusason.

Rognvald had received the favor of King Magnus the Good, who has granted him Brusi's share of the islands and the third which Olaf Haraldsson had claimed after Einar's death.

Thorfinn agrees to this division, but presents the transfer of the third claimed by the Norwegian king as a gift to Rognvald in return for aid in Thorfinn's wars in the Hebrides and the Irish Sea.

King Sigtrygg Silkbeard had died in 1035 or 1036, and the kingship in Dublin had come to Echmarcach mac Ragnaill, who in 1038 is challenged by Imar mac Arailt and driven out.

This instability in Dublin can only have helped Thorfinn and Rognvald, who raid far and wide and establish their rule over some part of the lands around the Irish Sea.

They are said to have won a major victory beside Vattenfjord, perhaps Loch Vatten on the west coast of the Isle of Skye.

The Earls are said to have raided in England, with mixed success.

Thorfinn and Rognvald had fallen out in time.

The vivid account of the war between Thorfinn and Rognvald in the Orkneyinga Saga which survives may well be only a part of a much longer saga now lost.

Their enmity arose with the arrival of Kalf Arnesson and his followers in Orkney.

Kalf is the uncle of Thorfinn's wife Ingibiorg Finnsdottir, and had left Norway to escape King Magnus Olafsson.

Rognvald, with Kalf's brothers, had shared Magnus's exile in Kievan Rus under the protection of Prince Yaroslav the Wise, and the saga says that when Kalf came to Ladoga to invite Magnus back to Norway, Rognvald almost attacked him.

Thorfinn, it is said, found hosting Kalf and his men a burden, and in time asked Rognvald to return the third of the earldom "which had once belonged to Einar Wry-Mouth".

Rognvald refused, saying that it was for King Magnus to settle matters.

Thorfinn began raising an army, and Rognvald's islanders were unwilling to fight Thorfinn, so Rognvald sailed to Norway where King Magnus supplied him with ships and men.

He returned to the islands, facing Thorfinn and Kalf Arnesson in a sea battle which Arnor the skald commemorated in verse.

The battle went Rognvald's way to begin with, but in the end he was defeated and forced again to seek refuge with King Magnus.

King Magnus offers to fit out another expedition for Rognvald, but he decides to take just one ship and a picked crew.

He sails to Shetland in winter, and learning that Thorfinn is staying on a farm on the Orkney Mainland with only a few men, he sets out at once to attack him.

Rognvald's men surprise Thorfinn, and set the farm ablaze.

The saga says that Thorfinn had to break down a wall and escape, carrying his wife in his arms, flying south to Caithness for safety.

Rognvald rules in Kirkwall over the winter, believing Thorfinn dead, but in the spring, while staying on Papa Stronsay, Thorfinn and his men turn the tables, taking Rognvald by surprise, just as he had surprised Thorfinn.

Rognvald escapes the house, but is tracked down, given away by the barking of his lap dog, and killed by Thorkell the Fosterer.

Even with Rognvald dead, Thorfinn is not entirely secure.

The saga recounts an attempt to make peace with Magnus Olafsson, who had sworn vengeance for the death of his men in Thorfinn's attack on Rognvald.

Magnus is at war with the Danish king Sweyn Estridsson, and dies before he can take any action.

Magnus's uncle and successor, Harald Sigurdsson, better known as Harald Hardrada, is more friendly towards Thorfinn, and makes a peace, accepting Thorfinn's gifts.

…the Orkney Islands in 1098.

The Norse jarls from this point forward twill owe allegiance both to Norway for Orkney and to the Scottish crown through their holdings as Mormaers of Caithness.

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.

The Norse jarls of Orkney, through their holdings as Earls of Caithness, have, unusually, owed allegiance both to Norway for Orkney and to the Scottish crown from about 1100 onward.

The line of Norse earls, unbroken since Rognvald, had ended in 1231 with Jon Haraldsson's murder in Thurs.

The Earldom of Caithness has been granted to Magnus, second son of the Earl of Angus, whom Haakon IV of Norway in 1236 confirms as Earl of Orkney.

The fifty-nine-year-old Haakon, called the Old, retreats to the Orkney Islands and, electing to winter in the Bishop's Palace at Kirkwall, falls ill. On his deathbed, Haakon declares that he only knows of one son who is still alive, Magnus, who subsequently succeeds him as King Magnus VI following Haakon’s death on December 17.

Magnus, if he does not already, will soon come to consider peace with the Scots more important than holding on to the Norwegian possessions off western Scotland and in the Irish Sea.

Eventually to be known as the Lawmender, Magnus had two years earlier married the Danish princess Ingeborg, the daughter of the late Danish king Erik Plogpenning, after she was practically abducted by Haakon's men from the monastery in which she was living.

The struggle to claim Ingeborg's inheritance from her murdered father will soon involve Norway in intermittent conflicts with Denmark for decades to come.