A Sea King was the Viking name for a powerful pirate.
They could be actual kings of Sweden (such as Erik Björnsson and Jorund), Denmark or a Norwegian kingdom, but they could also be men without roof like Hjörvard the Ylfing.
In the Icelandic sagas, such men without roof could be so powerful that they could subdue a country and make themselves kings. Two examples are Sölve who killed the Swedish king Östen, and Haki who killed the Swedish king Hugleik. However, in both cases they ultimately lost due to lack of popular support.
