Harald Bluetooth Gormson (Harald Blåtand, , Harald Blåtann), was born around 935, the son of King Gorm the Old and of Thyra (also known as Thyre Danebod) a supposed daughter of Harald Klak, Jarl of Jutland, or daughter of a noblemen of Sønderjylland who is supposed to have been kindly disposed towards Christianity. He died in 985 or 986 having ruled as King of Denmark from around 958 and king of Norway for a few years probably around 970. Some sources state that his son Sweyn forcibly deposed him as king.
The nickname Blátönn ("Bluetooth") could indicate that Harald had a "blue" or dark tooth, maybe stemming from an accident, but the epithet is probably a later invention.
In Old Norse, blá "blue" could also mean "black" - for instance, when the Vikings encountered black Africans, they were referred to as "blámenn", literally "blue men", their land being called "Blåland" = "Blue land".
Harald had the Jelling mounds – previously started by his pagan father Gorm – adapted into Christian monuments honoring both Gorm and Thyre. The Jelling monuments are said to have been a statement of Harald's new-found religion; it was thought that with these monuments, he was trying to conduct a smooth transition from paganism to Christianity both for himself and his subjects. Christianity may have been impressed on him as a result of military pressure, but the stones have led some people to believe that they represent a new-found love and confidence for his new religion.
Meanwhile the Christian religion became more and more deeply rooted among the Danes. Even a few members of the nobility (such as Frode, Viceroy of Jutland) embraced the faith and soon episcopal sees were established (Schleswig, Ribe, Aarhus). The first recorded attempt at Christianization was made by the English missionary Willibord in the early eighth century. The attempt was unsuccessful, but Willibord is said to have taken thirty young Danish men back to England, possibly to start a seminary. Other attempts were made after this time, but they too were largely unsuccessful. In 845 the Danes sacked Hamburg the town where Anskar, the Bishop of Hamburg, resided. As an indirect result of the sack, Anskar was compensated and given the richer see of Bremen which was run jointly with Hamburg.
It was not until 935 that Christian missionaries had a major breakthrough in the Christianization of Denmark. At this time the Archbishop of Hamburg-Bremen, Unni received Harald’s permission to begin preaching across Denmark, even though Harald was not yet the king.
However the prominent part the Germans had in these achievements as well as the lofty idea of the Roman Empire then prevailing led Otto I, the Great, to require Harald to recognize him as "advocatus", or lord protector of the Danish church, and even as "Lord Paramount". The king of the Danes replied to this demand with a declaration of war, and the emperor sought to force his "vassal" into subjection. The devastating expeditions, which were pushed as far as the Limfjord, enabled the emperor to beat down all opposition (972), and to compel Harald not only to conclude peace and submit to the emperor again. Henceforth paganism steadily lost ground.
In 974 Harald rebelled against emperor Otto II and the emperor took Schleswig. In 983 Harald supported a Slav uprising.
The Bishopric of Odense was established at Funen (Fyn) in 980; the sacrificial grove at Lethra (on Zealand), which, until then, had been from time to time the scene of human sacrifices, was deserted. King Harald moved his royal residence to Roskilde and erected there a wooden church dedicated to the Holy Trinity. In the eleventh century it was replaced by a basilica, which in turn was soon torn down. Since about the year 1200 its site has been occupied by the Gothic cathedral of St. Lucius, the burial place of the kings of Denmark. Christian houses of worship were also built in many other places during Harald's reign; in these German and Danish priests preached the gospel of the crucified and risen Saviour.
Harald undoubtedly professed Christianity at that time; it is also true that he contributed to its spread. But his moral conduct in many respects distinctly violated biblical commandments. This attitude toward Christianity can be seen throughout the Norse world. The Christian god became a part of Norse life, but was no more important, at first than their gods which already existed. A good example is the Jelling Stones made by Harald I. The rune-stone has both Christian and pagan qualities demonstrating the mixture of old and new values. Consequently many people looked on the plots that were directed against the sovereignty and life of the aging prince by his own son Svend as a punishment from Heaven. Although baptized, Svend joined forces with Palnatoke, the most powerful chieftain on Funen, who led the heathen party. The fortunes of war varied for a time, and Harold took refuge from his son, but finally Harald was slain on 1 November, 985 or 986.
He also constructed the oldest known bridge in southern Scandinavia, known as the Ravninge Bridge in Ravninge meadows, which was 5m wide and 760m long.
While absolute quiet prevailed throughout the interior, he was even able to turn his thoughts to foreign enterprises. Again and again he came to the help of Richard the Fearless of Normandy (in the years 945 and 963), while his son conquered Samland and, after the assassination of King Harald Graafeld of Norway, he also managed to force the people of that country into temporary subjection to himself.
The Norse sagas presents Harald in a rather negative light. He was forced twice to submit to the renegade Swedish prince Styrbjörn the Strong of the Jomsvikings- first by giving Styrbjörn a fleet and his daughter Tyra, the second time by giving up himself as hostage and an additional fleet. Styrbjörn brought this fleet to Uppsala in Sweden in order to claim the throne of Sweden. However, this time Harald broke his oath and fled with his Danes in order to avoid facing the Swedish army at the Battle of the Fýrisvellir.
As a consequence of Harald's army having lost to the Germans in the shadow of Danevirke in 974, he no longer had control of Norway and Germans having settled back into the border area between Scandinavia and Germany. The German settlers were driven out of Denmark in 983 by an alliance consisting of Obodrite soldiers and troops loyal to Harald. Soon after, Harald was killed fighting off a rebellion led by his son Sweyn. He was believed to have died in 986, although there are many other accounts that claim he died in 985.
He died 1 November, 985 or 986. His remains were buried in the cathedral at Roskilde, where his bones are still preserved, walled up in one of the pillars of the choir.