Short answers, pulled from the story.
The exact location of Jomsborg has not been established. The most widely proposed site is the modern town of Wolin, on Wolin island in northwestern Poland, where a multi-ethnic trading settlement existed in the Early Middle Ages. Other theories place it on now-submerged lands near Usedom island. The only source giving a precise location is the controversial Gesta Wulinensis ecclesiae pontificum, which situates Jomsborg near Paprotno, Poland.
Sources disagree on the founder of Jomsborg. The Knytlingasaga and Fagrskinna attribute its founding to Danish king Harald Bluetooth in the 960s, while the Jomsvikinga Saga names the Danish Viking Palnatoki as its founder. Harald Bluetooth lived from around 910 to 985 or 986.
The Jomsvikings were an elite brotherhood of selected warriors who lived at Jomsborg, adhered to a strict codex, and were loyal only to their own leader. Most records name Sigvaldi, son of petty king Strut-Harald of Scania, as their jarl. Sigvaldi died sometime before 1010.
According to the Heimskringla, Jomsborg was destroyed in 1043 by Dano-Norwegian king Magnus the Good. The fortress was burned down and many of its inhabitants were killed.
The Curmsun Disc is a golden disc weighing 25.23 grams that appeared in Sweden in autumn 2014, presented by Swedish archaeologist Sven Rosborn. Its Latin inscription translates as 'Harald Gormsson king of Danes, Scania, Jomsborg, diocese of Aldinburg,' making it the only physical object known to name Jomsborg directly. Historians Kurt Villads Jensen and Wojtek Jezierski have argued that the disc is a forgery, noting it has no known analogue and that an associated chronicle contains anachronisms.
The existence of the Jomsvikings remains debated. Historian Lauritz Weibull dismissed Jomsborg as a legend, and professor Steven Fanning has written that all attempts to locate the fortress have failed, leading many scholars to doubt the Jomsvikings existed outside of literature. Historian Wladyslaw Filipowiak, however, points to several dated sources attesting to armed Vikings in Wolin at the end of the 10th century.