Out of all the number of Titles a player can earn for their character in Sea of Thieves, Pirate Legend is the top known title and the ultimate goal for any player. Compared to every other known title, Pirate Legend, being that it is the greatest title of all, is the most difficult to obtain.
In order to become a Pirate Legend and rule the seas, you must do the near impossible and reach a Level 50 Reputation with not one, not two, but all three of the initial Trading Companies (Gold Hoarders, Merchant Alliance, Order of Souls). As you might guess, this will take any player a great amount of time in order to achieve. However, after accomplishing this, you can collect the exclusive title of Pirate Legend simply by donning all of your newly unlocked Pirate Legend Gear using the Vanity Chest.
Congratulations, you are now a king of the seven seas!