The Tyrants of the Black Ocean is a minor faction of Rogue Pirates introduced in the Aye-Aye! Patch for Total War: Warhammer II. They are one of the seven legendary pirate armies, which must be defeated for Vampire Coast factions to raise their Infamy rank.
Unlike the Rogue Armies associated with the Books of Nagash for the Tomb Kings, this army is always on the map whether you are playing as the Vampire Coast or not. Additionally, they can declare war on factions they encounter.
However, they will always stay at sea and patrol a specific route. As long as you steer clear of them they won't give you any trouble.
In the year VII, 183 (2483 IC), the Corsair captains Lokhir Fellheart and Dastan Coldeye challenged one another to a contest of plunder. Each chose a province of the Empire -- Ostland in Fellheart's case, and Nordland in Coldeye's -- and had a year to claim whatever bounty they could. Coldeye swiftly took the lead, thanks largely to the presence of an Empire fleet, its holds bursting with stolen Nehekharan treasures, at anchor in Debneitz when his Corsairs descended. Fellheart was not so easily bested, however, and drove his crews mercilessly until there was neither village nor town in northern Ostland that had not learned to fear said-crews. Nonetheless, a year's end approached, Fellheart still lagged behind, so he did what any Dark Elf does in his situation -- he cheated.
Fellheart sent word to King Akkateph of Zandri, informing him of the stolen Nehekharan treasures that now lay in Coldeye's possession. Akkatep's reaction was everything Fellheart could have wished for, and soon, Coldeye's Black Ark was pursued, harried, and ultimately sunk, by a fleet of Zandrian war dhows. Coldeye survived that battle, with with his vessel gone, and his treasure taken by Akkateph's fleet, had to cede the contest. At the next high moon, he and his immediate family were sacrificed to Mathlann and Loec, for Fellheart knew full well that his victory could have been achieved without the consent of the god of the sea and the god of deception.
This is the toughest Rogue Pirate army and should be avoided accordingly. The lord starts at rank 35 which should tell you plenty about whether or not you should pick a fight with them. Most of the army has gold chevrons as well, be well prepared when engaging.