Hades, Meet Podarkes
Simply kill Podarkes, the leader of the island and one of the many cultists you need to kill. Make him vulnerable by either completing some of the above quests – Kyra with a Cause & Bleeding Hearts and Stolen Money are good options. If he is ever overthrown, then he’ll be extremely vulnerable as he’ll have but a fraction of the guards he would normally have. Kill him to complete the quest and another target down in your quest to kill all the cultists.