Magnus Olafsson (Old Norse: Magnús Óláfsson, Norwegian and Danish: Magnus Olavsson; c. 1024 – 25 October 1047), better known as Magnus the Good (Old Norse: Magnús góði, Norwegian and Danish: Magnus den gode), was the King of Norway from 1035 and King of Denmark from 1042, ruling over both countries until his death in 1047.

He was an illegitimate son of Olaf II of Norway, but fled with his mother when his father was dethroned in 1028. He returned to Norway in 1035 and was crowned king at the age of 11. In 1042, he was also crowned king of Denmark. Magnus ruled the two countries until 1047, when he died under unclear circumstances. After his death, his kingdom was split between Harald Hardrada in Norway and Sweyn Estridsson in Denmark.

Magnus was an illegitimate son of King Olaf Haraldsson (later St. Olaf), by his English concubine Alfhild, originally a slave (thrall) of Olaf's queen Astrid Olofsdotter. Born prematurely, the child was weak and unable to breathe for the first few minutes, and he was probably not expected to survive. Olaf was not present at the child's birth, and his Icelandic skald Sigvatr Þórðarson became his godfather. In a hasty baptism, Sigvatr named Magnus after the greatest king he knew of, also Olaf's greatest role model, Karla Magnus, or Charlemagne. Against the odds, Magnus went on to grow strong and healthy, and he became of vital importance to Olaf as his only son.

Olaf was dethroned by the Danish king Cnut the Great in 1028, and he went into exile with his family and court, including the young Magnus. They travelled over the mountains and through Eidskog during the winter, entered Värmland, and were given shelter by a chieftain called Sigtrygg in Närke. After a few months, they departed Närke, and by March went eastwards towards Sigtuna, where the Swedish king Anund Jacob had left them a ship. The party thereafter sailed through the Baltic Sea and into the Gulf of Finland, eventually landing in Kievan Rus' (Garðaríki). They made their first stop at Staraya Ladoga (Aldeigjuborg) to organise the further journey. From there they travelled southwards to Novgorod (Holmgard), where Olaf sought assistance from Grand Prince Yaroslav the Wise. Yaroslav, however, did not want to become directly involved in the Scandinavian power-struggles, and declined to help. After some time, in early 1030, Olaf learned that the Earl of Lade Håkon Eiriksson, Cnut's regent in Norway, had disappeared at sea, and gathered his men to make a swift return to Norway. Magnus was left to be fostered by Yaroslav and his wife Ingegerd.

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