https://en.wikipedia.org/wiki/Victual_Brothers
The Victual Brothers were organised as a brotherhood or guild. Their main naval enemy in 1392 was the powerful Hanseatic town of Lübeck, which supported Denmark in the war. Apart from Lübeck, the Hanseatic League initially supported the Victual Brothers. Most of the Hanseatic towns had no desire for a victory for Denmark, with its strategic location for control of the seaways. For several years from 1392, the Victual Brothers were a strong power in the Baltic Sea. They had safe harbours in the cities of Rostock, Ribnitz, Wismar and Stralsund. They soon turned to open piracy and coastal plunder. In 1393 they sacked the town of Bergen for the first time and in 1394 they conquered Malmö. They occupied parts of Frisia and Schleswig. They also plundered Turku, Vyborg, Styresholm, Korsholm and Faxeholm castle at Söderhamn in Hälsingland.[4]
At the climax of their power, the Victual Brothers occupied the island of Gotland, Sweden, in 1394 and set up their headquarters in Visby. They also operated from the Turku archipelago; Knut Bosson, who was the chief of Turku Castle from 1395 to 1398, had allied himself with the people of Mecklenburg, which is why he supported the hijacking activities of the Victual Brothers and allowed them to operate in the area