Short answers, pulled from the story.
The Jomsvikings were a legendary order of Viking mercenaries active in the 10th and 11th centuries. They were reputed to fight for any lord who could pay their fees, regardless of religion, while adhering to a strict internal code of military conduct. Their stronghold, Jomsborg, was said to be on the southern shore of the Baltic Sea.
The exact location of Jomsborg has never been conclusively identified. The sagas place it on the southern shore of the Baltic Sea, and some researchers identify it with Jumne, Julin, or Vineta, sites mentioned in Danish and German medieval records near the modern Polish town of Wolin. The Curmsun Disc, a golden object re-discovered in Sweden in 2014, names Jomsborg directly but does not give a map coordinate.
Jomsviking code required members to be between 18 and 50 years old, prove their valor in a ritual duel to gain entry, defend and avenge their brothers, distribute all battle spoils equally, and never show fear before an equal or weaker enemy. Members were forbidden from quarreling, speaking ill of fellows, or leaving Jomsborg for more than three days without permission. No women or children were permitted inside the fortress walls.
The Jomsvikings fought at the Battle of the Fýrisvellir near Uppsala in 984 or 985, suffering defeat against King Eric the Victorious of Sweden. In 986 they were defeated again at the Battle of Hjörungavágr against Haakon Jarl of Norway. In the year 1000, a Jomsviking force led by Sigvald Jarl betrayed King Olaf of Norway at the Battle of Svolder, contributing to his fleet's destruction.
The Curmsun Disc is a golden object weighing 25.23 grams, re-discovered in Sweden in autumn 2014. Its Latin inscription names Harald Bluetooth as king and lists Jomsborg among his territories, making it one of the only physical objects to mention the Jomsviking stronghold directly. Researchers believe the disc originally came from a Viking hoard found in 1841 in the Polish village of Wiejkowo.
The Jomsvikings declined sharply after their defeats at Fýrisvellir in 984-985 and Hjörungavágr in 986. According to the Heimskringla, King Magnus I of Norway delivered the final blow in 1043, sacking Jomsborg as part of consolidating his control of Denmark, destroying the fortress and killing many of its inhabitants.