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.
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.