The Battle of Svolder (Svold or Swold) was a large naval battle during the Viking age, fought in September 1000 in the western Baltic Sea between King Olaf of Norway and an alliance of the Kings of Denmark and Sweden and Olaf's enemies in Norway. The backdrop of the battle was the unification of Norway into a single independent state after longstanding Danish efforts to control the country, combined with the spread of Christianity in Scandinavia.
https://en.wikipedia.org/wiki/Battle_of_Svolder