They appear in the campaign if the player is playing as a Vampire Coast faction. The player will be presented with a dilemma to choose one of four factions to go after for varying amounts of treasure.
Once you have selected a faction, they will spawn a single army which is hostile to you and begin to sail around the map. You will have a quest to kill them in order to get your reward. Note that another faction may kill them before you get the chance, causing you to fail your quest.
They use the Empire Rebels crest for their faction.
Strategy to deal with them:
Kill them before anyone else can. When they reach their destination they will disapear, so make sure to attack them before that.