To get this you have to be rank 50 in Merchants, Order of Souls and Gold Hoarders. Once you reach level 50 and purchase all the individual promotions from each faction in the top left "promotions" from each of the three faction vendors for leveling up mission difficulty you need to speak to the mysterious stranger in any bar. After they complete their rhyme you will become a pirate legend and givin the base pirate legend outfit and a new shanty to play on your instruments. Playing the new tune in the bar near the left wall will open a hidden stairwell in the floor that grants you and your crew access to the legends hideout.
edit: I did try to speak to the mysterious stranger without buying the final gold hoarder upgrade and he told me when I reach 50 to come back. I purchased last gold hoarder upgrade and spoke to him again and it popped right away.