Middle Sea Brigands is a minor faction of Rogue Pirates introduced in the Aye-Aye! Patch for Total War: Warhammer II. They are a special legendary pirate army, which will challenge Vampire Coast factions who raise their Infamy rank to 7. Defeating them unlocks a verse of the Sea Shanty.
They are grouped with Rogue Armies on the Diplomacy screen, though their crests all feature a notable teal stripe, as oppose to the red stripe generally seen on Rogue Armies or the blue stripe on regular Rogue Pirate armies.