Short answers, pulled from the story.
Cnut, also known as Canute the Great, was a Danish prince who became King of England from 1016, King of Denmark from 1018, and King of Norway from 1028 until his death on the 12th of November 1035. The three kingdoms he united are referred to by historians as the North Sea Empire.
Cnut sailed to England in the summer of 1015 with a fleet of 200 longships carrying an army of perhaps 10,000 men. After over a year of warfare against Edmund Ironside, the Battle of Assandun on the 18th of October 1016 proved decisive. Cnut and Edmund then divided England by treaty; Edmund died on the 30th of November 1016, and the entire realm passed to Cnut under the terms of that agreement.
Cnut had been baptized a Christian under the name Lambert, and he worked to reconcile himself with the Church by repairing monasteries and churches damaged in Viking raids, making gifts to sees across England and Europe, and travelling to Rome in 1027 to attend the coronation of the Holy Roman Emperor. He also negotiated a reduction in the fees paid by English archbishops for their pallium.
The story was first recorded by Henry of Huntingdon in his Historia Anglorum in the early 12th century. Cnut ordered his chair placed on the seashore and commanded the tide not to rise, then allowed the sea to drench his feet and shins before declaring that the power of kings is "empty and worthless" compared to the authority of God. In modern readings, the story is interpreted as a deliberate demonstration of royal humility, not an act of delusion.
Cnut had two wives: Ælfgifu of Northampton, by whom he had Sweyn Knutsson (King of Norway) and Harold Harefoot (King of England); and Emma of Normandy, widow of Æthelred, by whom he had Harthacnut (King of Denmark and England) and Gunhilda of Denmark, who married Henry III, Holy Roman Emperor. Cnut wed Emma in July 1017.
Cnut died on the 12th of November 1035 in Shaftesbury, Dorset, and his empire quickly fragmented. Harold Harefoot ruled England until 1040, after which Harthacnut briefly reunited Denmark and England until his own death in 1042. Cnut's daughter Gunhilda died in Italy before she could become empress consort, ending the potential tie to the Holy Roman Empire.