Den of Thieves
 

Where can I find it? That's all I need to know, thanks!

I think it's a cave in Tortuga. Go into a forest and find a cave. It should be called Thieves Den.

As shown in my illustrated guide to tortuga , if you enter the cave entrance by Fabiola (tortuga graveyard) turn right and head down to the bottom of the hill, the entrance to the Thieves Den is on your right at the bottom.

Do check out that pictured guide for better instructions on finding your way around this most important of islands in the game.

Good luck.