Thorfinn Sigurdsson, also known as Thorfinn the Mighty and Earl Thorfinn lived from about 1009 to 1064. He rose to become Earl of Orkney and control Orkney, Shetland, and the Hebrides, plus Caithness and Sutherland. The wider picture in Scotland at the time is set out in our Historical Timeline.
Thorfinn Sigurdsson was the youngest of five sons of Sigurd Hlodvirsson, Earl of Orkney, but the only one whose mother was the Earl's second wife, one of the daughters of Malcolm II of Scotland. As a result, he was Malcolm II's grandson. Earl Sigurd was killed in battle in Ireland in 1014, and Orkney was divided between three of Thorfinn's older brothers, Sumarlidi, Einar and Brusi. Malcolm II responded by granted Caithness and Sutherland to the young Thorfinn with the title Earl of Caithness, and by arranging his fostering by Thorkel Amundason.
Sumarlidi died in about 1015 and the still very young Thorfinn Sigurdsson claimed Sumarlidi's third of Orkney. This led to conflict with his brother Einar, who wanted Sumarlidi's share for himself. Armies were raised by Einar on Orkney and on Thorfinn's behalf in Caithness, but war was avoided when Earl Brusi persuaded Einar to accept a compromise that saw Thorfinn gain a third share of the islands. Einar continued to cause problems, however, and made life extremely difficult for Thorkel Amundason, who was acting as Thorfinn's tax collector on Orkney. Thorfinn and Thorkel then gained support from King Olaf Haraldsson of Norway. Armed conflict was again avoided by the intervention of Earl Brusi, but during the festivities to cement the new accord, Thorkel Amundason killed Einar.
In 1021 Thorfinn and Brusi appealed to King Olaf of Norway to help resolve a dispute between them over the division of Einar's share of Orkney. This he did, largely by taking a third for himself, to be managed by Brusi. When King Olaf was overthrown in 1028, Brusi handed over King Olaf's third of Orkney to Thorfinn. Thorfinn then took charge of Brusi's remaining third on the latter's death in 1030, leaving the still only 21 year old Earl Thorfinn as the undisputed holder of the Jarldom of Orkney from the King of Norway and the the Earldom of Caithness from the King of Scots.
Some time in the 1030s, Thorfinn's hold on the Earldom of Caithness seems to have been challenged by the Earl of Ross. He successfully fought off this challenge, and reportedly responded by raiding deep into Scotland. In 1037 King Magnus I of Norway granted the third of Orkney the Norwegians believed they owned to Thorfinn's nephew Rognvald Brusason, arguably in gratitude to Rognvald's support for Thorfinn's successful campaigns of conquest through the Hebrides and in Ireland. Rognvald and Thorfinn subsequently fell out, and after Thorfinn had been surprised and almost killed at a farm on Orkney's Mainland, he subsequently turned the tables and killed Rognvald on the island of Papa Stronsay. While all of this was going on, Earl Thorfinn found the time and the forces he needed to join with one cousin, Macbeth, to help him challenge another cousin, King Duncan I, for the Scottish throne. The three met in battle on 15 August 1040 near Elgin; Duncan I was killed, and Macbeth became King of Alba.
By upbringing, Thorfinn was a Christian. He oversaw the introduction of the first Bishop of Orkney to the islands in 1035. Then, in 1048, Thorfinn went on a pilgrimage to Rome. The date of Thorfinn's death has been the subject of some controversy. According to some sources he must have died in the early 1050s, because his widow Ingibiorg is recorded as having then married Malcolm III of Scotland. Other sources claim he died in about 1064 and the Ingibiorg who married Malcolm III must have been Thorfinn's daughter. If all this sounds a little vague, remember that this was an era when only very selective accounts of events were published: the uncertainty is such that there are even people who have suggested that Thorfinn the Mighty and Macbeth were actually one and the same person. Thorfinn was buried in the grounds of the Christchurch he built at Birsay, on Orkney, since replaced by St Magnus Church.