This is still marked as "(secret) on TSA. The description for winning is "You caught a tailwind." After you unlock The Path Is Clear by defeating the first guardian of the islands, you will unlock tailwinds. Tailwinds are bonuses to additional playthroughs in the roguelike, side-story mode of the DLC. Speak to the navigator and retire your current character. Start a new character. (at level one, with little gear) Before creation, you will be asked to chose a tailwind. You can refuse. Pick a bonus and complete character creation to unlock this achievement.
As an example, I picked "Dwarves get +1 Con for each Dwarf in the party." That was +6 constitution once I filled my party with all Dwarven women. You should unlock stronger tailwinds by completing more islands.