Short answers, pulled from the story.
The Jomsvikings were a legendary order of Viking mercenaries or conquerors active during the 10th and 11th centuries. Their story appears in Icelandic sagas from the 12th and 13th centuries including the Jómsvíkinga saga and King Olaf Tryggvasson's Saga.
Texts claim their stronghold of Jomsborg stood on the southern shore of the Baltic Sea but modern historians have never agreed on its exact location. Some researchers suggest it might be identical with Jumne Julin or Vineta mentioned in Danish and German records from the Middle Ages.
Membership in the Jomsviking order was restricted to men of proven valor between 18 and 50 years old. A boy named Vagn Åkesson defeated Sigvaldi Strut-Haraldsson in single combat at the age of 12 as an exception to this rule.
This conflict occurred at the Battle of Fýrisvellir near Uppsala in either 984 or 985 during the early 980s when exiled Swedish prince Styrbjörn the Strong brought the Jomsvikings to a devastating defeat against his uncle King Eric the Victorious.
The Curmsun Disc was re-discovered in Sweden during autumn 2014 and bears the name of Harald Bluetooth identifying Jomsborg as a stronghold seat. This golden disc has a weight of 25.23 grams and contains high gold content with a Latin inscription reading +ARALD CVRMSVN+REX AD TANER+SCON+JVMN+CIV ALDIN+.
King Magnus I of Norway decided to put an end to the Jomsviking threat according to Heimskringla and sacked Jomsborg in 1043. The fortress was destroyed and many inhabitants were killed during this attack which marked the beginning of their decline over the next few decades.